From Diablo Wiki
The Marquise Gem is the level 15 gem, a new, highest level added to the game in Patch v1.0.7. These gems are very expensive to create and socket, and are BoA, thus can not be sold or traded or even given away. Players must teach their Jeweler to make the Marquise gem of the appropriate type (the designs *can* be sold) from 3 Radiant Star gems, plus 20 million gold and 10 Demonic Essences.
Prior to patch v1.07, there were only 14 levels of gems, the three highest of which had to be found or bought as Jeweler designs, while the first eleven levels were learned by the Jeweler via his training level. The Marquise gem adds slightly better stats, though the function of the gem is the same in all cases.
Marquise gems are considerably more expensive than other games to create and even to unsocket, and they are designed largely as a gold sink to bleed off the profits of rich players. Players with less wealth will generally wish to not create Marquise gems, since the cost is far higher than the benefit, compared to what other levels of gems add for the gold required to create them. From the official patch notes:
- 3 Radiant Star Gems
- 20 million gold
- 10 Demonic Essence
Un-socketing a Marquise gems costs 5 million gold.
- The designs for the Marquise gems will drop randomly from all level 63 monsters
Note that Marquise Gems (and any item with a Marquise gem socketed in it) is Account Bound and can not be sold or given away.
The total price to create a Marquise gem includes all the lower levels of gems that go into it.
To create a Radiant Star gem costs:
- 15.4m in Jeweler fees
- 1631 Tomes of Secret
- 729 Flawless Square gems
A Marquise gem requires 3 Radiant Stars, plus additional gold, so the total cost of a Marquise Gem is:
- 15.4m x 3 + 20m = 66.2 million gold in Jeweler fees.
- 1631 ToS x 3 = 4893 Tomes of Secret
- 729 FS gems x 3 = 2187 Flawless Square Gems
- 10 Demonic Essence
Of course most players rich enough to use Marquise Gems aren't scrounging up the raw ingredients and sitting there clicking the Jeweler's crafting bar for an hour. They are simply buying 3 Radiant Star gems on the Auction House and then crafting those into a Marquise Gem.
 Marquise Gem Total Economics
There's no way to pinpoint the actual total cost then, since the prices for gems and Tomes of Secret vary widely across time, between patches, and especially between realms and Softcore vs. Hardcore. A quick comparison from a month after the v1.07 patch introduced Marquise Gems:
March 2013 Americas Softcore:
- Tomes of Secret: 400g x 4893 = 1.95m gold
- Flawless Square gems: 700-5500g (Amethyst cheapest, Emerald most expensive) x 2187 = 1.5m - 12m
- Or simply buying 3 Radiant Star Gems: 21m (63m for 3) for Topaz/Ruby/Amethyst, 26m for Emerald (78m for 3)
March 2013 Americas Hardcore:
- Tomes of Secret: 12,000g.
- Flawless Square gems: 220-7700g. (Topaz are cheapest, Amethyst by far the most expensive. Emeralds are only 1200g, 1/3 their price in SC.)
- Or simply buying 3 Radiant Star Gems: Amethyst 75.4m (x3 = 226.2m), Emerald 68m (x3 = 204m), Ruby 72m (x3 = 219m), Topaz 51m (x3 = 153m).
Thus a Marquise Amethyst would cost about 82m in Softcore vs. 247m in Hardcore, thanks to +life being very valued in Hardcore, and the vastly higher costs of all the materials to create the item, with Tomes of Secret the biggest price difference. That's why the most expensive gem in Softcore costs less than half what the least expensive does in Hardcore.
 Marquise Gem Functions
The properties granted by Marquise Gems are as follows:
- Weapon: Increased Critical Hit Damage by 110%
- Helm: +33% Extra Gold from Monsters
- Armor: +62 Dexterity
- Weapon: Melee attackers take 2500 per hit
- Helm: 33% Better chance of finding magical items
- Armor: +62 Intelligence
- Weapon: Each Hit adds +700 Life
- Helm: +19% Life
- Armor: +62 Vitality
- Weapon: +160 Minimum and +160 Maximum Damage
- Helm: Increases Bonus Experience by 33%
- Armor: +62 Strength
Note that the function of all levels of Rubies in weapons were turned up Patch 1.0.7 to attempted to make them more comparable to the very high value of Emeralds in weapons.
|Items of Diablo III  Item Basics Normal Items Legendary Armor I Legendary Armor II Legendary Weapons 1h Legendary Weapons 2h Item Sets|
|Item Basics||Item Modifiers||Inventory||Artisans||Merchants||Miscellaneous |
|Armor - Class-Specific||Weapons - Class-Specific||Crafting Items||Other Items |
|| • Blacksmith Plans
|Waist||Wrists & Hands||Legs & Feet||Head |
|| Voodoo Masks - WD|
| Chest Armor
|| Amulets cont.
|| Rings cont.
40) Puzzle Ring
|| Sources - Wiz|
|| Fist Weapons - Monk
|| Mighty Weapons - Barb|
|| Daibos - Monk
|| Bows |
|Item Sets |