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...
			





