Button plant.

That is one of several common names. Its proper name is Conophytum. These grow in South Africa and Namibia. They are really small. The flower in the photo is has the same diameter as a £1 coin.
There are lots of different species. I bought this one last Thursday when we visited the large Cactus nursery in Matlock, Derbyshire. In fact the National Collection of Conophytums is held there.

They are not to be confused with Lithops often known as living stones although one can see the similarities.

I think I may buy a few more and grow them in a bowl with some pebbles.