Every gemstone grade below S is still sold by the Mineral Traders you find in each town. S-Grade Gemstones are different: they are cut by hand by a Gemcutter, and there is exactly one.
Where to find him
| NPC | Title | Town |
|---|---|---|
| Gedrik | Gemcutter | Goddard Castle Town |
What he cuts
| Item | Price |
|---|---|
 Gemstone: S-Grade |  20,000 Ancient Adena +  65,000 Adena |
Talk to him and choose Cut S-Grade Gemstones. He works from Ancient Adena, so bring it with you -- the Mineral Traders will not take your order, and their own list now stops at A-Grade.
Gedrik is also a quest NPC. If you are running the Saga of the Ghost Sentinel, his Quest link is on the same page.