A Massive guide to desynthing

Source: http://forum.square-enix.com/ffxiv/t...-Endgame-Guide

Links
Follow this link for a Google Doc that can help calculate the cost of desynthing various items. To use it, make a copy of it so you can edit it. Then, on the Desynth Skill tab, enter your dSkill in each class. On the Raw Materials tab, enter the MB prices of each Raw Material in the MB field and leave the rest alone. On the Desynth Items tab, you can see the prices of desynthing each item, which tells you what's cheapest to use to level or get demimateria. The database only has ~500 items in it. To add a new item, enter the crafting materials as shown, the desynth class, the clvl (recipe level) of the item, and the number of materia slots it has. Sort the Desynth Items tab as needed to get the info you're looking for.

The wordy version of my guide has been moved to this document. The rest will be more concise.

For you CUL desynthers out there, check out this site to see what fish desynth into what. The important items are also listed in the CUL section of this guide.

General Notes
Each materia melded on an item increases the chance that desynthing it will succeed by 5%. This makes i70 and i90 crafted equipment items particularly attractive for leveling your desynth skill, though still quite expensive.

High Quality items do not have any better chances of giving demimateria. However, if you're melding materia to an item to increase desynth chances, it's worth it to know that HQ items are easier to meld to (~10% boost).

You can only get one type of non-crystal item back from desynthing, but you can get multiple of these items, up to a maximum of the number it takes to craft it. If an item requires 3x Darksteel Nuggets but nothing else to craft it, you can get back 1, 2, or 3 Darksteel Nuggets from desynthing it. If instead it takes 1x Darksteel Nugget, 1x Rose Gold Nugget, and 1x Spruce Plywood, you can only ever get a single one of these back, and only 1x of it. This means that items with fewer types of materials are generally cheaper to use when leveling than items with many types of materials.

Desynthing weapons that have non-materials like equipment or parts in the recipe cannot give back the equipment or parts used. Instead, the materials used to make the parts are added to the final product's recipe when determining what you get back.

In a future patch, food and equipment that increases desynthing chances will be introduced. (link)

When you level up a desynth skill, there is a chance of losing 0.01 experience in another desynth class that you've previously leveled. If you hit the 300 point combined cap, you will lose as much points in other skills as you gain in the skill you are leveling. However, there is a trick to controlling which class you lose the points in whenever you lose points. See this thread for details.

Comparisons

Mastercraft Demimateria

Best: GSM, BSM, ARM
Mid: WVR
Worst: CRP
Can't Get: LTW, ALC, CUL(?)

Fieldcraft Demimateria III

Best: BSM, GSM, CRP
Mid: WVR
Worst: ALC, LTW, ARM
Can't Get: CUL

Battlecraft Demimateria III

Best: ARM, LTW, WVR
Mid: GSM
Worst: BSM, CRP, ALC
Can't Get: CUL

Demimateria of the Inferno

Best: BSM
Mid: ALC, CRP
Worst: GSM
Can't Get: CUL, ARM, LTW, WVR

Demimateria of the Vortex

Best: BSM
Mid: GSM, ALC, CRP
Can't Get: CUL, ARM, LTW, WVR
Demimateria of Crags
Best: ALC
Can't Get: CUL, BSM, GSM, CRP, ARM, LTW, WVR

Demimog Demimateria

Best: BSM, ALC
Mid: CRP
Worst: LTW
Can't Get: CUL, ARM, WVR, GSM

Classes

GSM
Arguably the easiest class to level and the best to have for getting demimateria, since accessories tend to require fewer mats to craft than other pieces (excluding i90 ones). Has i90 options for both Fieldcraft IIIs and Battlecraft IIIs.

Leveling Guide

dSkill 1-10: Rusty Needle (i1) [90% - 100%] (50 gil each from Ul'Dah vendor)
dSkill 10-20: Copper Needle (i8) [91.8% - 100%] (184 gil each from Ul'Dah vendor)
dSkill 20-30: Batfang Needle (i16) [87% - 100%] (buy all mats from vendors FC house or Limsa/Ul'Dah)
dSkill 30-36: Initiate's Needle (i24) [86.8% - 100%] (buy mats for mats from FC house vendors, or buy mats from MB, or buy needles for 1k each)
dSkill 36-46: Sandworm Needle (i32) [69.6% - 91.2%] (1x Fieldcraft Is)
dSkill 42-55: Electrum Needle (i47) [40% - 66%] (1-2x Fieldcraft Is)
dSkill 42-55: Tortoiseshell Armillae (i49) [36% - 62%] (1-2x Battlecraft Is)
dSkill 55-75: Aetheryte Ring (i55) [50% - 85%]
dSkill 70-85: Darklight accessories (i70) [50%-77.5%] (2x each from side chests in 2.2 dungeons (Halitali HM, Bray HM, Lost City)
dSkill 75-95: Quad-melded Gold Spectacles (i90) [42.5% - 80%] (assuming high price of Peacock Ore (> 10k) and low price of Raziqsand (< 20k)
dSkill 95-100: Triple-melded Gold Spectacles (i90) [75% - 85%] (assuming high price of Emery (> 10k) and low price of Raziqsand (< 15k)
dSkill 87-100: Carbuncle Chronometers (i90) [44% - 70%] (assuming moderate price of Raziqsand (> 15k) and low price of Emery (< 10k)

GSM Endgame Items

Darklight accessories (i70) -> Rose Gold Ingot, Battlecraft Demimateria III
Rose Gold Clasps -> 1 Fieldcraft Demimateria III
Artisan's Spectacles -> 1 Mastercraft Demimateria,1 Rose Gold Ingot, 1 Fieldcraft Demimateria III
Gold Spectacles -> 1-2 Battlecraft Demimateria III (estimated yield: 0.20)
Carbuncle Chronometer -> 1-2 Fieldcraft Demimateria III (estimated yield: 0.20)
Ifrit's Cudgel [BLM] -> 1-3 Demimateria of the Inferno
Garuda's Van [BLM], Wile [WHM] -> 1-3 Demimateria of the Vortex
Crown of Light [PLD/WAR], Royal Crown [WHM/SCH] -> (unknown)
Ultima Bands/Chokers -> (unknown)
Divine Accessories from the 2.3 dungeons -> (unknown)
South Seas Talisman -> Lapis Lazuli?

BSM
BSM has the advantage of being able to desynth a number of Primal (non-Titan) weapons, Summoning Bells (wonderful at 100 skill for getting Fieldcraft IIIs), and Artisan's/Forager's tools (great way of getting Mastercraft Demimateria). Unfortunately, BSM is not a good way to get Battlecraft IIIs unless you love farming HM Primals.

Leveling Guide

dSkill 1-10: Weathered Shortsword (i1) [90% - 100%] (buy for 63 gil in Limsa Lominsa)
dSkill 10-20: Amateur's Culinary Knife (i6) [98.6% - 100%] (buy for 136 gil in Limsa Lominsa)
dSkill 20-30: Plumed Bronze Pickaxe (i16) [87% - 100%] (buy for 511 gil in Ul'dah) {probably something cheaper here}
dSkill 30-36: Initiate's Mortar (i24) [86.8% - 100%] (buy mats for 244 gil from vendors and synth for ~500 gil)
dSkill 36-40: Plumed Iron Hatchet (i29) [83.2% - 93%]
dSkill 40-50: Plumed Steel Hatchet (i35) [72% - 93%]
dSkill 50-54: Mythril File (i40) [77% - 83.6%] (buy the mythril ingots from the Amal'ja vendor) {probably better choices here}
dSkill 54-64: Horned Hatchet (i49) [60% - 77.5%] (Fieldcraft I, push this as far as you can)
dSkill 64-75: Militia Scythe (i55) [68% - 85%]
dSKill 75-80: Garuda (HM) PLD, WAR, MNK weapons (i70) [60%-70%]
dSKill 75-85: Moogle (HM) PLD, WAR, DRG weapons (i75) [50%-70%]
dSkill 75-100: Summoning Bell (i90) [22.5% - 70%] (assuming high price of Coke (> 10k) and low price of Scheelite and Petrified Logs (< 20k total)

BSM Endgame Items

i70 Crafted Weapon parts -> 1 Fieldcraft Demimateria III
i70 Crafted Weapons -> 1 Battlecraft Demimateria III
Warwolf Blade -> 1x Battlecraft Demimateria III
Artisan/Foragers Tools -> 3 Mastercraft Demimateria, 1 Fieldcraft Demimateria III
Summoning Bell -> 1-2 Fieldcraft Demimateria III
Ifrit's Blade [PLD], Axe [WAR], Claws [MNK], Harpoon [DRG] -> 1-3 Demimateria of the Inferno
Garuda's Gaze [PLD], Scream [WAR], Talons [MNK] -> 1-3 Demimateria of the Vortex, 1 Battlecraft Demimateria III
Morbid Mogblade [PLD], Malignant Mogaxe [WAR], Melancholy Mogfork [DRG] -> 1-3 Demimog Demimateria, 1x Battlecraft Demimateria III

CRP
CRP is, in my opinion, one of the least interesting desynth classes. CRP does have a fairly easy and cheap i70 furniture desynth (Straight Stepping Stones), and a few options for i90 furniture. However, CRP is probably the worst class in the game for getting Battlecraft IIIs (apart from CUL, of course).

Leveling Guide (incomplete, will finish later)

dSkill 1-10: Weathered Fishing Pole (i1) [90% - 100%] (buy for 74 gil in Limsa Lominsa)
dSkill 10-20: (i5) []
dSkill 20-30: (i15) []
dSkill 30-36: (i26) []
dSkill 36-46: (i34) []
dSkill 46-55: (i41) []
dSkill 46-55: (i48) []
dSkill 55-75: Militia Grinding Wheel (i55) [50% - 85%] (Fieldcraft II) (Credit: Lyrinn Loriae of Ultros)
dSkill 55-75: Militia Spinning Wheel (i55) [50% - 85%] (Fieldcraft II) (Credit: Lyrinn Loriae of Ultros)
dSkill 55-75: Wolf Guisarme (i55) [50% - 85%] (Battlecraft II)
dSkill 65-75: Ifrit (HM) BRD, WHM weapons (i60) [60% - 77.5%] (Battlecraft III, Inferno)
dSkill 70-85: Straight Stepping Stumps (i70) [50% - 77.5%] (Fieldcraft IIIs)
dSKill 75-80: Garuda (HM) BRD, DRG weapons (i70) [60%-70%] (Battlecraft III, Vortex)
dSKill 75-85: Moogle (HM) BRD, BLM weapons (i75) [50%-70%] (Battlecraft III, Demimog)
dSkill 80-100: Carbuncle Chair (i90) [30% - 70%] (Fieldcraft III)

CRP Endgame Items

Straight Stepping Stumps -> 1 Fieldcraft Demimateria III
Carbuncle Chair -> 1-2 Fieldcraft Demimateria III
Ifrit's Bow [BRD], Cane [WHM] -> 1-3 Demimateria of the Inferno
Garuda's Spine [BRD], Beak [DRG] -> 1-3 Demimateria of the Vortex, 1 Battlecraft Demimateria III
Maleficent Mogbow [BRD], Mogstaff [BLM] -> 1-3 Demimog Demimateria, 1 Battlecraft Demimateria III

WVR
Very cheap to level in the beginning, but less so later on. Good options for Fieldcraft IIIs, and moderately good options for Battlecraft IIIs.

Leveling Guide

dSkill 1-18: Hempen Bandana (i5) [75.6% - 100%] (buy for 85 from city vendors)
dSkill 18-30: Cotton Halfgloves (i16) [81% - 100%] (make them for ~275, or buy them for 336 from city vendors)
dSkill 30-36: Velveteen Turban (i26) [75.4% - 91%] (buy all mats from Sylphic Vendor)
dSkill 36-46: Linen Halfgloves (i33) [67.2% - 89.4%] (buy all mats from Sylphic Vendor)
dSkill 46-55: Linen Deerstalker (i40) [68.2% - 85.3%] (buy all mats from Sylphic Vendor)
dSkill 55-70: Weaver's Gibus (i55) [50% - 77.5%] (other options available - this was cheapest for me)
dSkill 70-80: Riviera/Glade/Oasis Cushion (i70) [50% - 70%] (assuming cheap Waterfowl Feathers)
dSkill 80-95: Quad-melded Cashmere Hat of Casting (i90) [60% - 80%]
dSkill 95-100: Triple-melded Cashmere Hat of Casting (i90) [75% - 85%]

WVR Endgame Items

Riviera/Glade/Oasis Cushion -> 1 Fieldcraft Demimateria III
Carbuncle Round Table -> 1-2 Fieldcraft Demimateria III
Cashmere Hat of Casting -> 1-2 Battlecraft Demimateria III
Forager's Hat -> 1 Mastercraft Demimateria, 1 Fieldcraft Demimateria III
16 items from LotA -> (unknown)
9 items of Divine gear from the 2.3 dungeons -> (unknown)
Sea Breeze Summer Halter, Sea Breeze Pareo, Blue Summer Maro -> Waterproof Cotton
Coronal Summer Halter, Red Pareo, Tei Leaf Lei, Red Summer Maro -> (unknown)

LTW
LTW has one of the best sources of Battlecraft Demimateria IIIs, but no easy way to get Fieldcraft Demimateria IIIs and no way at all to get Primal demimateria (except Demimog).

Leveling Guide

dSkill 1-18: Leather Choker (i5) [75.6% - 100%] (buy for 88 gil from city vendors)
dSkill 18-30: Hard Leather Choker (i15) [89.6% - 100%] (buy for 294 gil from city vendors)
dSkill 30-36: Goatskin Choker (i26) [75.4% - 91%] (mats should be really cheap, if not, 879 gil from city vendors)
dSkill 36-40: Toadskin Brais (i29) [83.2% - 93%]
dSkill 40-46: Toadskin Duckbills (i35) [72% - 85.8%] (depends on relative prices of Toad Leather vs Boar Leather - another good choice are Boarskin Ringbands. Note: you can buy the Undyed Linen or Steel Ingot from Beast Tribe vendors)
dSkill 46-53: Boarskin Tool Belt (i40) [68.2% - 82%]
dSkill 53-60: Raptorskin Ring (i48) [60% - 73%] (Fieldcraft I) (Credit: Dystia Wexnia of Lamia)
dSkill 60-75: Militia Choker (i55) [60% - 85%] (Battlecraft II)
dSkill 60-75: Leatherworker's Hat (i55) [60% - 85%] (Fieldcraft II)
dSkill 60-75: Weaver's Gloves (i55) [60% - 85%] (Fieldcraft II)
dSkill 75-80: Triple-melded Saurian Belt of Aiming (i90) [37.5% - 45%]
dSkill 80-95: Double-melded Saurian Belt of Aiming (i90) [40% - 70%]
dSkill 95-100: Single-melded Saurian Belt of Aiming (i90) [65% - 75%]

LTW Endgame Items

Carbuncle Armchair -> 1-2 Fieldcraft Demimateria III
Gryphonskin Shoulder Guards/Shin Guards/Elbow Pads/Knee Pads -> 1 Fieldcraft Demimateria III
Saurian Belt of Aiming -> 1-2 Battlecraft Demimateria III
Murderous Mogfists [MNK] -> 1-3 Demimog Demimateria
8 items from LotA -> (unknown)
16 items of Divine gear from the 2.3 dungeons -> (unknown)

ARM
Like LTW, ARM is great for getting Battlecraft IIIs. It is, however, one of the worst classes in the game for getting Fieldcraft IIIs. It has a decent ability to get Mastercraft Demimateria (via desynthing Artisan's Frypans), but, like all the other armor-making classes, cannot get Primal demimateria.

Leveling Guide

dSkill 1-10: Weathered Skillet (i1) [90% - 100%] (66 gil each from Limsa vendor)
dSkill 10-20: Bronze Sallet (i8) [91.8% - 100%] (buy mats from vendors and craft for ~250 gil, or buy for 283 gil from vendor)
dSkill 20-30: Iron Skillet (i15) [89.6% - 100%] (craft for ~400 gil or buy for 528 gil from vendor)
dSkill 30-36: Iron Alembic (i23) [89.6% - 100%] (buy mats from vendors and craft for ~530 gil)
dSkill 36-45: White Skillet (i33) [67.2% - 87.6%] (buy Steel Plate from Amal'ja vendor)
dSkill 45-50: Steel Frypan (i36) [74.8% - 83.6%] (buy Steel Plate from Amal'ja vendor)
dSkill 50-55: Cobalt Skillet (i43) [64% - 73%] (Fieldcraft Is)
dSkill 52-57: Hell's Kitchen (i47) [60% - 70%] (Fieldcraft Is)
dSkill 55-65: Thermal Alembic (i50) [60% - 77.5%] (Fieldcraft Is)
dSkill 60-75: Wolf Scale Fingers (i55) [60% - 85%] (Battlecraft IIs) (Credit: Alistaire Lexander of Sargatanas)
dSkill 60-75: Blacksmith's Boots (i55) [60% - 85%] (Fieldcraft IIs) (Credit: Alistaire Lexander of Sargatanas)
dSkill 70-80: Warwolf Kite Shield (i70) [50% - 70%] (410 Wolf Marks) (Credit: Gelesto T'ki of Moogle)
dSkill 75-80: Triple-melded Wolfram Tassets (i90) [37.5% - 45%] (assuming Scheelite + Terminus Putty < 20k)
dSkill 80-95: Double-melded Wolfram Tassets (i90) [40% - 70%]
dSkill 95-100: Single-melded Wolfram Tassets (i90) [65% - 75%]

ARM Endgame Items

Artisan's Frypan -> 3 Mastercraft Demimateria, 1 Fieldcraft Demimateria III
Warwolf Kite shield -> 1 Battlecraft Demimateria III (cheapest equipment in the game that is purchasable with Wolf Marks, but unmeldable)
Wolfram Tassets -> 1-2 Battlecraft Demimateria III
10 items from LotA -> (unknown)
9 items of Divine gear from the 2.3 dungeons -> (unknown)

ALC
ALC desynth is the only way to get Demimateria of Crags. It is, however, arguably the worst for getting Fieldcraft Demimateria IIIs, as it has no i90 furniture nor i90 equipment. It does have the cheapest way to get Battlecraft Demimateria IIs, but they're of limited usefulness.

ALC Leveling Guide (incomplete, will finish later)

dSkill 1-10: Weathered Grimoire (i1) [90% - 100%] (buy for 92 gil in Limsa Lominsa)
dSkill 10-20: (i8)
dSkill 20-30: (i16)
dSkill 30-36: (i26)
dSkill 36-42: (i33)
dSkill 42-48: (i40)
dSkill 48-55: (i48)
dSkill 55-75: Madman's Whispering Rod (i55) [50% - 85%] (Credit: Lyrinn Loriae of Ultros)
dSkill 60-70: Ifrit (HM) SMN, SCH weapons (i60) [50% - 70%] (Credit: Lyrinn Loriae of Ultros)
dSkill 70-80: Garuda (HM) SMN, SCH weapons (i70) [50% - 70%] (Credit: Lyrinn Loriae of Ultros)
dSkill 70-80: Giantsgall weapons (i70) [50% - 70%] (Credit: Lyrinn Loriae of Ultros)
dSkill 75-85: Mog (HM) SMN, SCH, WHM weapons (i75) [50% - 70%] (Credit: Lyrinn Loriae of Ultros)
dSkill 80-100: All Titan HM drops* (i80) [50% - 85%] (Credit: Lyrinn Loriae of Ultros)

[* I recommend you get a team of all friends/FC mates together to farm Titan HM for a while. Have everyone else who isn't leveling ALC desynth pass on the loot, and divvy up the Demimateria of Crags you get from them.]

ALC Endgame Items

Armor Augmentation items -> 1 Battlecraft Demimateria III
Suspended Trillium Flower -> 1 Fieldcraft Demimateria III
Giantsgall Weapons -> 1 Battlecraft Demimateria III
i70 SCH/SMN books -> 1 Battlecraft Demimateria III
Ifrit's Codex [SCH], Grimoire [SMN] -> 1-3 Demimateria of the Inferno
Garuda's Embrace [SCH], Will [SMN] -> 1-3 Demimateria of the Vortex
All of Titan's HM drops -> 1-3 Demimateria of Crags
Malevolent Mogwand [WHM], the Mogtomes [SCH/SMN] -> 1-3 Demimog Demimateria

CUL
The only items that CUL can desynth are fish. At endgame, CUL can desynth Big Fish to get the twelve swimwear-related items (Sea Breeze Pareo, Sea Breeze Summer Halter, South Seas Talisman, Blue Summer Maro, Waterproof Cotton, Coronal Summer Halter, Ti Leaf Lei, Red Summer Pareo, Red Summer Maro, Summer Sandals, Gold Roselle Capeline, and Oschon Roselle Capeline). Unfortunately, CUL seems to be a one-trick pony, and that trick is swimsuits.

Note that fish can never desynth into any Demimateria except Clear Demimateria, which exists only to be sold to vendors for gil. Fish can desynth into shards and crystals (but never clusters). All fish can give water shards/crystals. Sand fish can also give earth shards/crystals, sky fish can also give wind crystals, lightning-related fish (such as the Judgeray) can also give lightning crystals, and fiery fish (Bronze Lake Trout and Joan of Trout) can also give fire shards/crystals. No fish yet desynths into ice shards/crystals.

A trick to getting a specific fish faster from a fishing hole is to look at how your rod moves when you hook a fish. There are three types of tugs, as far as I can see. A very light tug where your rod barely moves, a moderately strong tug where the end of your rod bends noticeably (about 45 degrees) and makes a stretching noise, and a massive tug that bends your rod over 90 degrees and makes a hollow, wooden noise (and your controller will go crazy, even if you're in another window (Credit: Estellios Deatras of Hyperion)).

The same fish always triggers the same kind of tug, but there's only a weak correlation between tug strength and ilvl for the first two tug types. That last, massive tug type means you definitely have a Big Fish on your line (though a few Big Fish are only moderate tugs). If you hook a fish you don't want, wait for the fish to go away and cast again (or cancel and cast again). This burns through bait quickly, but if you're using lures, you only go through ~5 an hour or so.

This trick more than doubled my Silver Sovereign fishing rate. It also vastly increased my Octomammoth catch rate since only two fish pulled up by Pill Bigs are moderate tugs, and if my mooch off Harbor Herring doesn't trigger a massive tug, it's not an Octomammoth. I highly recommend trying this.

Leveling Guide

dSkill 1-30: Buy increasingly expensive fish from Holasfhis (i2-i15) [86.4% - 100%] (Limsa Lominsa, SW of Aftcastle Aethernet Shard)
dSkill 25-35: Copperfish (i20) [90% - 100%] (100% catch: Southern Thanalan (Forgotten Springs), Midge Basket (mooch for Lamprey (i29))
dSkill 35-45: Lamp Marimo (i31) [69.6% - 91.2%] (100% catch: Coerthas (Exploratory Fishing Hole), Chocobo Fly)
dSkill 35-50: Fullmoon Sardine (i34) [62.4% - 94.8%] (100% catch: Western La Noscea (South Umbra Isles), Iron Jig, 6am-6pm (mooch for Coelacanth (i55) or Nautilus (i45))
dSkill 40-50: Giant Bass (i37) [61.6% - 82%] (100% catch: East Shroud (Sylphlands), Silver Spoon Lure)
dSkill 45-55: Agelyss Carp (i46) [48% - 68%] (75% catch: Eastern La Noscea (East Agelyss River), Topwater Frog)
dSkill 55-70: Gaint Squid (i55) [50% - 77.5%] (100% catch + 100% Mooch + 100% Double Mooch: Western La Noscea (Ship Graveyeard), Floating Minnow)
dSkill 55-70: Coelacanth (i55) [50% - 77.5%] (100% catch + 50% Mooch: Western La Noscea (South Umbra Isles), Iron Jig->Fullmoon Sardines, 6am-6pm)
dSkill 55-70: Any i55 fish (there are many options here) [50% - 77.5%]
dSkill 70-85: Coeurlfish (i70) [50% - 77.5%] (40% catch + 100% Mooch: Upper La Noscea (Oakwood), Spinner->Common Sculpin (light tug))
dSkill 70-85: Gigantshark (i70) [50% - 77.5%] (63% catch + ?%** Mooch: Middle La Noscea (Summerford), Floating Minnow->Harbor Herring (moderate tug), Clear/Fair weather)
dSkill 70-85: Octomammoth (i70) [50% - 77.5%] (36.6% catch + ?%** Mooch: Limsa Lominsa Lower Decks, Pillbug->Harbor Herring (moderate tug), 9am-5pm)
dSkill 70-85: Any i70 fish (there are many options here) [60% - 77.5%]
dSkill 80-90: Silver Sovereign* (i75) [60% - 77.5%] (6% catch: Lower La Noscea (Oschon's Torch), Yumizumo (massive tug))
dSkill 85-100: Twitchbeard (i80) [60% - 77.5%] (?%** catch + ?%** Mooch: Mist (Mist), Spoonworm->Fullmoon Sardine (moderate tug), 4am-6am, Clear/Fair weather)
dSkill 85-100: Shark Tuna (i80) [60% - 77.5%] (?%** catch + ?%** Mooch: Eastern La Noscea (Costa del Sol), Spoonworm->Fullmoon Sardine (moderate tug), 7pm-9pm, Clear/Fair weather)
dSkill 85-100: Any i80 fish (there are many options here) [60% - 77.5%]

[* The indicated fish are untradable]
[** Catch rate indeterminate due to weather and/or time restrictions skewing the data]

(Starting at 35 skill (or earlier), I highly recommend grinding on Fullmoon Sardines at South Umbra Isles at night with an Iron Jig as bait. You will have a 100% chance to catch them, and if you get an HQ fish, you can Mooch it to either an i45 fish or an i55 fish. Once you have a fair number of the i55 fish and/or you have stopped getting CUL experience from the NQ Fullmoon Sardines, finish with the sardines and go ahead and break all the Nautiluses you Mooched up. Once they're all broken, if your skill is ~55+, go ahead and break the Coelacanths you also obtained through Mooching. This should be able to get you from 35 to 60 pretty fast.)

CUL Endgame Items

Gigantshark (i70, gathering: <308) -> Sea Breeze Summer Halter (7.7%), South Seas Talisman (12%)
Octomammoth (i70, gathering: <308) -> Sea Breeze Pareo (7.7%), Blue Summer Maro (7.7%)
Frilled Shark (i80, gathering: >379) -> Coronal Summer Halter, Ti Leaf Lei
Twitchbeard (i80, gathering <348) -> Red Summer Pareo, Red Summer Maro
Shark Tuna (i80, gathering: <348) -> Summer Sandals
Toramafish (i80) -> Gold Roselle Capeline
The Greatest Bream in the World (i80) -> Oschon Roselle Capeline
Olgoi-Khorkoi* (i80, gathering: <348) -> Waterproof Cotton (7.7%)
The Lone Ripper (i80) -> Waterproof Cotton
Armorer (i75) -> Ferberite
The Warden's Wand (i80) -> Native Gold
Jacques the Snipper (i75) -> Aqueous Whetstone
Chirurgeon (i70) -> Aqueous Whetstone
Titanic Sawfish (i70) -> Spruce Lumber [reports say they can drop Mastercraft Demimateria, but I believe that to be a hoax]
Silver Sovereign* (i75) -> Venture
Goldenfin (i70) -> Venture
Other Big Fish -> unknown (at least one was reported to yield minium)

[* The indicated fish are untradable]

Cheap sources of Demimateria
In this section, I'll try to list the theoretically cheapest desynths for each desynth class to get each type of Demimateria. Please note that these vary wildly depending on server prices and I'm using data from Faerie to make these calculations.

Following each item name will be [i#|#s]. The first number tells the recipe level of the item. The second number tells how many materia slots the item has. Together, these let you know the item's chance of successful desynth.

Included below are the expected values of the gil per demimateria calculation for some of the items at 50, 75, and 100 skill. The prices are being made using current data from Faerie, and are assuming you get the item's materials from the MB, a vendor, or from crafting it yourself (whichever is cheapest) and you make the item yourself. Sometimes, melding materia to the item will decrease the expected cost per demimateria yield more than it increases the cost of the desynth. In these cases, the number of melds will be listed. For the price of the melds, I am using 650 gil per meld attempt (500 is the price of the cheapest garbage materia on Faerie, and 150 is the current price of Carbonized Matter).

[ Warning: Section in progress; also, need to do research on Demimateria Is before definitively saying what works (both minimum ilvl and whether ilvl affects chances are unknown, but data strongly suggests that ilvl does matter) ]

Fieldcraft Demimateria I - Used to get the new crafting books (5x each), for crafting DoL/DoH accessories (10x), glamour items (5x), and furniture (2x)
[To obtain: Desynth ilvl 25?-49 DoL/DoH gear, furniture, and equipment parts (1x from i26?-i35?, 2x from i36?-i50)]

GSM: Electrum Needle [i47] (Yield: 0.333) - Crafted: 2,925 (gil/demimateria: GSM 77+: 8,775)
WVR: Linen Deerstalker [i40] (Yield: 0.167) - Crafted: 1,000; Vendor: 2,195 (gil/demimateria: WVR 64+: 6,000)
WVR: Linen Halfgloves [i33] - Crafted: 1,000; Vendor: 2,195
ARM: Steel Skillet [i31] - Crafted: 1,100; Vendor: 3115
GSM: Silver Ring of Crafting [i27] - Crafted: 1,100; Vendor: 1,254
GSM: Electrum Ring of Crafting [i45] - Crafted: ?
GSM: Thousand Needle [i43] - Crafted: ?
LTW: Raptorskin Ring [i48] - Crafted: ?
LTW: Raptorskin Merchant's Purse [i45] - Crafted: ?

Fieldcraft Demimateria II - Used for crafting Crescent Moon equipment (5x each) and new furniture (2-3x)
[To obtain: Desynth ilvl 55 DoL/DoH gear, furniture, and equipment parts (2x from i55)]
[i55 items appear to have a 10% chance of dropping 1 of these, and a 10% chance of dropping 2, giving an expected yield of 0.3 per successful desynth]

GSM: Aetheryte Ring [i55] - Crafted: 3,252 (gil/demimateria: GSM 50: 27,443; GSM 75: 12,753; GSM 100: 10,840)
WVR: Weaver's Gibus [i55] - Crafted: 5,713 (gil/demimateria: WVR 50: 47,226 (2 melds); WVR 75: 22,403; WVR 100: 19,043)
ARM: Blacksmith's Workboots [i55] - Crafted: 5,860 (gil/demimateria: ARM 50: 48,216 (2 melds); ARM 75: 22,980; ARM 100: 19,533)
LTW: Weaver's Gloves [i55] - Crafted: 6,000
CRP: Militia Grinding Wheel [i55] - Crafted: ?
CRP: Militia Spinning Wheel [i55] - Crafted: ?
BSM: Militia Sledgehammer [i55] - Crafted: 7,436
BSM: Militia Culinary Knife [i55] - Crafted: 7,085
ALC: Potted Dragon Tree [i55] - Crafted: ?

Fieldcraft Demimateria III - Used to get the new crafting tools (10x each) and for crafting new furniture (1-3x)
[To obtain: Desynth ilvl 70+ DoL/DoH gear, furniture, and equipment parts (1x from i70+ (Yield: ~0.133), 2x from i90+ (Yield: ~0.167)]

BSM: Summoning Bell [i90|0s] - Crafted: 27,742 (gil/demimateria: BSM 75: 739,788; BSM 80: 554,838; BSM 85: 416,010; BSM 90: 332,904; BSM 95: 277,422; BSM 100: 237,788)
GSM: Carbuncle Chronometer [i90|0s] - Crafted: ?
CRP: Carbuncle Chair [i90|0s] - Crafted: ?
LTW: Carbuncle Armchair [i90|0s] - Crafted: ?
WVR: Carbuncle Round Table [i90|0s] - Crafted: ?
CRP: Straight Stepping Stumps [i70|0s] - Crafted: 38,200 (gil/demimateria: CRP 75: 477,502; CRP 100: 286,500)
WVR: Riviera/Glade/Oasis Cushion [i70|0s] - Crafted: ? (takes demimateria + rare unspoiled BTN but no myth/philo mats)
GSM: Artisan's Spectacles [i70|0s] - Other: 68,060 gil + huge time sink (gil/demimateria: GSM 75: 850,747; GSM 80: 729,217; GSM 100: 510,451)
ALC: Suspended Trillium Flower [i70|0s] - Crafted: ?
ARM: Darksteel i70 armor parts [i70|0s] - Crafted: ?
LTW: Gryphonskin i70 armor parts [i70|0s] - Crafted: ?

Battlecraft Demimateria I - Used to get the new crafting books (5x each) and for crafting the i90 accessories (5x)
[To obtain: Desynth ilvl 25?-49 DoW/DoM gear (1x from ?-?, 2x from ?-i49]

WVR: Velveteen Turban/Rope Belt - Crafted: 900 (single mat type)
GSM: Tortoiseshell Armillae - Crafted: 1,300 (single mat type)
GSM: Electrum Ring [i44|1s] - Crafted: ?
WVR: Felt Bracers?

Battlecraft Demimateria II - Used for crafting augmented i90 crafted armor (10x) and i90 accessories (2x)
[To obtain: Desynth ilvl 55 DoW/DoM gear] (2x from i55)

LTW: Wolf/Militia Duckbills [i55|2s] - Crafted: ?
WVR: Wolf Tights [i55|2s] - Crafted: 6,000
ARM: Militia Cuirass/Armet [i55|2s] - Crafted: ?
CRP: Wolf Guisarmes [i55|2s] - Crafted: ?
ALC: Madman's Whispering Rod [i55|3s] - Crafted: 1,973
GSM: Militia Earrings [i55|1s] - Crafted: 5,929
GSM: Imperial Operative Wristlets [i55|1s] - Crafted: 7,756 (only one material type)
BSM: Wolf Winglet [i55|2s] - Crafted: ?

Battlecraft Demimateria III - Used for crafting upgraded Primal weapons (10x) and augmented i90 crafted armor (1x)
[To obtain: Desynth ilvl 70+ DoW/DoM gear (1x from i70+ (Yield: ~0.133), 2x from i90+ (Yield: ~0.167)]

LTW: Saurian Belt of Aiming [i90|1s] - Crafted: 24,339 (LTW 75: 79,636/chance (3 melds), LTW 82: 60,486/chance (2 melds))
ARM: Wolfram Tassets [i90|1s] - Crafted: 31,721
GSM: Gold Spectacles [i90|2s] - Crafted: 31,628 (gil/demimateria: GSM 75: 88,949/chance (4 melds); GSM 80: 75,607/chance (4 melds); GSM 100: 40,651/chance (4 melds))
WVR: Cashmere Hat of Casting [i90|2s] - Crafted: 32,298
LTW: Saurian Gloves of Casting [i90|2s] - Crafted: 30,983
ALC: Giantsgall Weapons [i70|1-2s] - Crafted: ?
ALC: i70 SCH/SMN books [i70|2s] - Crafted: ?
BSM: i70 Crafted Weapons [i70|2s] - Crafted: ?
CRP: Spruce Pattens [i70|2s] - Crafted: ?
GSM: Darklight accessories [i70|0s] - Other: 2.2 dungeon side chests (2x per dungeon)
ARM: Warwolf Kite Shield [i70|0s] - Other: 410 Wolf Marks
BSM: Warwolf Blade [i70|2s] - Other: 965 Wolf Marks

Mastercraft Demimateria - Used for getting the new i75 DoL/DoH tools (3x for each except FSH, which only takes 1x)
[To obtain: Desynth Artisan and Forager items (1x from hats, 3x from tools)]

GSM: Artisan's Spectacles [i70|0s] - Other: 68,060 gil + huge time sink (1x only, chance=~15%)
ARM: Artisan's Frypan [i70|0s] - Other: 340,300 gil + massive time sink (1-3x)
WVR: Forager's Hat [i70|0s] - Other: Time (if you have a 50 MIN and BTN and play ~2 hours a day, you can get 2 hats every 5 days)
BSM: Forager's Hatchet/Dolabra [i70|0s] - Other: Time (if you have a 50 MIN and BTN and play ~2 hours a day (the gathering itself only takes a few seconds), you can get 2 (one of each) every 10 days) (1-3x)
CRP: Forager's Rod [i70|0s] - Other: Time (it takes about 40 hours of constantly fishing to get one of these... horrible source of Mastercraft Demimateria)
ALC: No known options available
LTW: No known options available