Most reptile products are made of plastic or a light material and will float. I bought a fake rock dish for the aquarium floor but there was no way of keeping it on the bottom of the tank.
I cant say i have experience using these, but its a pretty safe bet that these are plastic and will not stay down.
Actually, I found it on the company's own site ... their photo there looks like it might be sandstone. That'll break down in the aquarium, but it takes a LONG time and isn't likely to cause much issue. I'd still do an acid test.
I went online, and according to one of the websites I found them on they are nontoxic, dishwasher safe, won't bleed or fade, and are made of 60% renewable resources. So...I'd think that they're probably not actually stone, but some other material, but if it's nontoxic, and a lot of reptiles also have water in their habitats, I'd think they'd be safe for fish. :)
If nothing else, as long as they are nontoxic, then I don't believe there can be anything in them that could hurt the fish.