I prefer knapping old TV picture tubes to bottle bottoms. The glass is about an inch thick, which makes any curve easy to remove. I can never seem to get a flat point from a bottle.
As far as homemade metal points go, the best raw material I've found is old stainless steel spoons. Pound them flat and cold forge/work them into the desired shape and sharpen. I'm always running into people with silverware they don't want, but even if you had to buy a few they're cheap at the bargain stores. If you're dead set on casting, look into bronze alloy casting grain/chunk. They have a bunch of different ones to choose from at most jewelry supply companies. Aluminium might be okay, but I wouldn't bother with lead or copper as are pretty soft. Sterling silver would probably work as well, but you can't beat stainless for durability.