Lithops, commonly known as Living Stone (or sometimes 'Pebble Plants'), are a genus of succulents that are native to the deserts of South Africa.
They're very small and grow incredibly slowly - they're unlikely to ever exceed 1 inch (2.5 cm) in height, and as they mature they form groupings of small 'pebbles' that can make an attractive display in their pot.
As a desert plant, Living Stones have adapted to be able to tolerate harsh conditions such as full sun, high temperatures and periods of drought. This makes them very low-maintenance, easy-care plants, as they can thrive well on neglect.
They should preferably get around 5 hours of direct sun during the day, but if you live in a hotter climate you may need to protect your Living Stone from the harsh midday sun, which may scorch its foliage. They can tolerate temperatures up to a maximum of around 100 °F (38 °C) but will do best at temperatures of around 65 - 80 °F (18 - 27 °C).
As they are able to store water in their fleshy leaves for prolonged periods of time, they can go quite a while without being watered again. It's much easier to kill a Living Stone by overwatering it than by underwatering it, as they can be prone to developing root rot which is often fatal. If you're unsure, you can take a look at the leaves to see if your plant needs more water. When a Living Stone is thirsty, it will start to look a bit wrinkly and deflated. You can also try very gently squeezing a leaf - a well-watered Living Stone should feel firm, but one that needs a drink will feel a bit softer than usual.
Living Stones consist of only a pair of leaves, and they grow a new set every year or so. Don't worry if you start to notice older leaves dying off - this is totally normal! It's just part of the cycle where they are replaced by new, fresh leaves. The plant absorbs the nutrients from the old leaves and uses these to provide the energy to produce new ones.
If your Living Stone starts to look stretched out or misshapen (known as 'etiolation'), this is likely a sign that it's not getting enough light. Try to provide your plant with more light, either by moving it to a sunnier spot or adding grow lights, and then keep an eye on your plant. Remember though that Living Stones have a very slow growth rate, so it may take a while for them to recover and return to their original size / shape.