Lithops is a genus of succulent plants from the family Mesembranthemaceae, endemic to arid regions of Southern Africa. Some common names include "Living Stones", or "Split Rocks", though these names can also apply to other genera of "mesembs", like Lapidaria, Pleiospilos or Argyroderma.
If given proper care, lithops can survive for a significant length of time and provide a great deal of entertainment throughout their lifecycle. The best way to care for your lithops is to try and replicate its natural environment as closely as possible.
Use a well-draining cactus soil with plenty of grit material. Or, make your own using 50% potting soil and 50% grit material (pumice and lava are best, perlite is a secondary option). Top-dress your potted lithops with a fine-grade gravel or rock to prevent winter rot.
Lithops grow naturally in full-sun, so light is important to bring out their vibrant colors. In coastal areas, lithops can be planted outside in all-day sun. Give them some afternoon shade in hotter, inland areas if outside. Lithops can grow surprisingly well indoors as well, but it is important to make sure they receive adequate light to avoid etiolation and color loss. East-facing windowsills are a good option (West-facing windows get too much radiant heat). If your plant begins to stretch inside, gradually re-introduce it to outdoor exposure.
This is where most people go wrong. Lithops come from extremely arid regions where rainfall totals average less than an inch per month. Therefore, lithops are naturally drought-tolerant and require little water. Water should be withheld completely in the winter. Water very sparingly in summer, as most species undergo a period of summer dormancy and will crack, split or burst if given too much. When the plant begins to appear shriveled, give it a drink to replenish its stock of water (best done in the morning and not during the heat of the day). Fall and spring are the growing months for Lithops, so this is when you want to water more frequently, but still no more than once every 10 days. Allow your soil to dry between waterings.
Most lithops bloom toward the end of summer or fall, and produce a daisy-like flower typical of the mesemb family. Flower colors include white, yellow, orange and red. After flowering, the plant will slowly split open, and a new plant will form out of its old "shell". The outside layer will eventually wither away.
Lithops produce a long tap root, so don't plant them in too shallow a pot. Protect your lithops from freezing temps, as frost will cause the plant to rupture and rot.