I'll sell it. Even if I did care about jewelry, I can buy a lot more jewelry than one diamond with $100,000 :P Besides, I can get diamond rings or necklaces for way less than that too if I'm concerned with wanting diamonds. It works out better for me to sell it.
It depends, if its a Tiffany Co. diamond or something then i'd sell it, but lets say its a diamond thats like vintage or couture and there's only very few in the world so its extremely hard to get your hands on, then i'd keep it, not only the price could exceed in the future, but rare items are so much better than money, which there is so much of, plus inflation is going up, so its only going to get more common.
I am not a Huge fan of Jewelry here, dear, and for Me... Diamonds are Not a girl's best friend. I probably would try and take to the nearest pawn shop and see what he would give me for the trouble for my bauble. xx
Keep it for a while, finish school and then sell it in order to start a company or my own resort. I'm underage and if I sell it now there's probably gonna be my mom using the money to pay bills, go on vacation and give money to family which is a good thing but not a great investment and I'd probably end up with nothing.
Personally I would hand it in to the nearest police station.. For the simple reason it could very well be of huge sentimental value to the owner.. Not only that I simply could never live with my conscience otherwise.