I would call it at a 100 gems per pound unless you specifically state it is a certain size (like the eyes of the Player Handbook Idol). Gems are weighed in carats and the problem is that the value per carat varies between gem types. Harnmaster is the only Fantasy RPG I know of that gives that information. Even armed with such information I feel it strays over the line of too realistic of a rule. Harnmaster rates each gem type with a price factor and then multiplies it by the weight in carats squared. Note that a carat is 2 grams. 227 carats in a pound (I rounded up).
A 100 per pound is a nice average that reflect gem's compact value at a somewhat accurate weight without having to go through a lot of bookkeeping. For those 1,000 gp, 5,000 gp gems just make up a weight from 1/4 to 1 pound.