Few more nice crystals grew today, this time it is Rochelle salt, or sodium potassium tartrate tetrahydrate: $$NaK C_4H_6O_6 \cdot 4H_2O.$$
This compound is very well known among crystal growers because of its availability and ability to form big and clear crystals easily, so I won't say much about the growing process. The usual method with slow evaporation was used, with seed crystal suspended on a thin thread. The crystals on the photo grew only in 3 weeks.
Rochelle salt can be prepared from the cream of tartar (unfortunately, it is not sold in my country, so I ordered some on eBay), and baking soda. Some instructions on preparation can be found on the Dom's crystal growing page (UPD: the site is down, but web archive have it), though I've simplified that recipe by using baking soda directly, without the heating step. Simply stir some cream of tartar powder in hot water and add small portions of baking soda until it stops fizzing. The reaction is: $$ K H C_4H_6O_6 + NaHCO_3 \rightarrow K Na C_4H_6O_6 + H_2O + CO_2 \uparrow.$$
This compound is not toxic and, in fact, edible (but not tasty at all). I would not recommend consuming big quantities though.
More photos of these crystals are in the gallery Crystal growing: Rochelle salt.
Update: piezoelectric effect
Rochelle salt is well known for its property to generate electricity, when mechanical force is applied to the crystal: piezoeffect. Actually, the voltage is so big that it can make flash a light emitting diode, directly connected to the crystal.
I took 2 pieces of aluminum foil and tightly attached them to the crystal sides with thread and adhesive tape. Wires are connected to these foil pieces with another piece of the same tape, and a pair of LEDs in opposite direction is connected to wires.
Then, in the dark room, I can see dim but visible red flashes, when the crystal is hit along the vertical axis (blue arrow on the schematic). To protect crystal from smashing, I put a small wooden piece between the crystal and the "hammer" (actually teaspoon).
So cool. Not that I ever doubted the existence of this effect, but it is really pleasure to observe such effects on a crystals, grown "from scratch", at the corner of the table.