These are "stone plants" (lithos in Greek means stone). The entire Aizoaceae (or Mesembryanthemaceae depending on who you ask) family is composed of succulent plants that developed this kind of mimicry to thwart herbivores from consuming them. They originate from South Africa.

I bought these plants from stores around here in L.A. (at around $1.50 each). Not all of them are Lithops but since that is the most known genus I stuck with it. I made the classification by comparing photos of known species so it may not be entirely accurate. If you know different please drop me a line.