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.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.