Drink coasters....I bought myself a set of them a few months ago that are made of ceramic with a cork bottom. I've been using them all winter, and they are great for hot cups of tea. What I'm waiting to see is how they perform in the summertime when my cold drinks will sweat from all the ice I put in them.
That being said, this new set I just got (at a discount) is made of diatomaceous earth (diatomite stone) and the seller says they will absorb liquids and let them evaporate. I hoping they really do, because that's what you want to happen. Coasters that are sealed or that don't absorb will just allow liquids to run off of them onto the surface you're trying to protect.
These are really pretty in person. They're super lightweight. They feel like ceramic, but they're actually compressed d.e. They can be sanded if they discolor any. They can be washed, but you need to make sure you allow them to air dry.
I am really looking forward to trying them!