Ooooh, our first crystal from the "Dangerous Crystal Experiment" (from Dangerous Book for Boys).
Ironically nothing grew so far on the "starter rock" hanging from the string. But there are a whole bunch of little crystals like this one (looks big but it is only about 1cm across).
It is basically pyramid-shaped on the top and comes to a really nice point. It doesn't have much index of refraction or "color", though the glow of the computer screen shows nicely in the picture. It is almost perfectly clear, but under 30x magnification you can see some nice surface details/defects. It feels really fragile and light and I am pretty sure I could crush it back to powder without much effort.
Why a pyramid? I need to review some basic Chemistry for this one. Anyone need a cheap gem? Just don't get it wet.