I'm sure its been sugjested before but searching turns up nothing for Gems so I assume old threads have been nuked. Basically a new class of minables that get used in jewlery (either modifying the existing jewlery recipies or for whole new ones). Gems are ofcourse very very deep being mined at a low probability ( <10%) from their own type of vein, on failure a regular stone is produced, on success a rough gem is placed in inventory. The rough gem must be cut before its usable and this requires use of a new Steel gem cutting tool the quality of which acts to cap the cut gems quality in the same manor a gold beater skins dose for jewlery. Cutting has a failure rate like that of smelting modified in the same way by nature/industry, in total an average of 100 mining actions are needed for 1 gem at nutral Ind/Nat. Possible gems include Diamond, Emeralds, Rubies and Saphires.
On a side note Amber might be a usefull and period apropriate gem, Baltic amber was a major gemstone in the middle ages and most comes from Königsberg in Prussia (A place Jorb seems to have some strong feelings about). I think Amber apearing as a very rare and hard to see Foragable on Sand would be good. Amber is actualy collected on the shore ware it washes up from the bottom of the ocean, and currently the game lacks any kind of foragable that spawns in Sand.