On the AH it seems like the cost to buy some of the gems outright is cheaper than it is to combine 3 lesser gems, but I don't have any to check. How much does it cost (in gold and materials) to combine gems between each quality level?
I've listed all of the costs below. Remember, that this is the cost in addition to the gems of the previous level.
The recipes below require 2 gems of the previous level
- Rank 1 Chipped: n/a
- Rank 2 Flawed: 10 gold
- Rank 3 Regular: 25 gold
- Rank 4 Flawless: 40 gold
- Rank 5 Perfect: 55 gold and 1 Page of Jewelcrafting
- Rank 6 Radiant: 70 gold and 2 Pages of Jewelcrafting
- Rank 7 Square: 80 gold 1 Tome of Jewelcrafting
- Rank 8 Flawless Square: 100 gold and 2 Tomes of Jewelcrafting
The gems below do not drop and must be crafted using 3 gems of the previous level
- Rank 9 Perfect Square: 30,000 gold and 3 Tomes of Secrets
- Rank 10 Radiant Square: 50,000 gold and 6 Tomes of Secrets
- Rank 11 Star: 80,000 gold and 9 Tomes of Secrets
- Rank 12 Flawless Star: 100,000 gold and 12 Tomes of Secrets
- Rank 13 Perfect Star: 200,000 gold and 15 Tomes of Secrets
- Rank 14 Radiant Star: 400,000 gold and 20 Tomes of Secrets
Thankfully, Gems in Diablo III are reusable, so you won't have to recreate Rank 14 gems every time you get a new socketed upgrade.