Perhaps a Gemstone could be worth .5(1/2) of a credit to fill a need in game for a partial credit.
Maybe a technology in Academy can allow mining to begin.
Of course the appropriate hiring costs and wages must be paid to miners to reflect the true cost of a gem and these costs should reflect what 1/2 a credit is worth.
How they get out into the market ?
Perhaps sell them through your banker ?
Banker can resell to the public or try to sell them yourself for larger profits
The cost to make and the selling price should allow a banker to make a small profit and the producer make a small profit. Bankers buying in bulk should get a better rate than single purchases.
Now I am rambling...