It's a truly silly name and one that had us giggling like six year olds when we spotted them in the Garden Centre as we were leaving the Lost Gardens of Heligan. They really are quite peculiar. And I'd never seen or heard of these before, have you?
So their proper name (or genus) is Lithops, which isn't anywhere near as funny or as juvenile.
And as you'd expect there's more information about them on Wikipedia. Here's a quick summary of what I've discovered: - there's hardly any stem, it's just the leaves that are visible - they have beautiful daisy-like flowers (see the bottom picture - no pun intended) - they occur naturally across Namibia and South Africa and in parts of Botswana and Angola but haven't naturalised outside of those countries they can be grown as house plants but need very little water and often none at all during winter