Originally posted by Neo-Rio:

I did place it on top of earth (leveled with my hoe), but no luck with that.
I'll try placing it on stone later, and see if that works in the context of a wood house.

See if the hearth is actually on top of the earth and not on top of any surrounding items. For example, remove the immediate surrounding wooden floors, and then place the hearth on top of the earth, and put the wooden floors back in after. If it still breaks, then I'm at a loss.