I don't think it's too big a deal if it's a bit off. They're usually an ellongated rectangular polygon. For the tip, you may like to pick a center point, at any spot really, and draw connecting lines to each of the corners. try to make it look pyramid shaped at the tip. For a more advanced tip, you could try picturing a cross section in the center, and dividing the end into four smaller squares that end at different lengths.
Other than that, they're basically a transparent white.

I searched the #quartz tag,but that wasn't really useful. Maybe you could google search 'quartz gem' for a better reference?