The "living stone" plants - in the family Aizoaceae.

The idea of plants like that that "fade into the background" in their native habitats is a fascinating one.  The idea is for them to avoid being browsed.  A short growing season, plus little or no frost, makes succulence an ideal adaptation in particular desert habitats.  If a plant is faced with a short wet season followed by a long dry season, what better adaptation than to have leaves that are tanks of water?  But the peril that must be overcome is that leaves like that are ideal water and food sources for browsing animals.  Hence the evolution of resemblance to rocks.  And particular species do tend to match the rock backgrounds in which they grow.

The Stone Plants have evolved to survive in extremely harsh environments in very arid and hot conditions, are found in South Africa and comprise of the following main genus: Lithops, Conophytums, Agryoderma, Pleiospilos, Frithia, Fenestaria and Faucaria. Each genera has developed its own methods of survival. Lithops, as an example, have windows in the skin to allow light to enter the inner surface of the leaf (much like your eyeball does). The photosynthetic tissue is located on the inner surface allowing the leaf to be completely sealed during the day to avoid releasing water into the atmosphere. Its main structure lies below ground level, with only the window exposed to conserve moisture.

This is so efficient that it has been know to survive several years without a re-hydration. Many of the stone plants have is the ability to recycle their own fluids. Example: Lithops consists of a single pair of fleshy leaves. When developing a new pair, it draws the fluids and nutrients from the old leaves until they have totally withered. Thus development is achieved without the need for new any water source.