I did a little testing and it seems to be a wall-like solid (in fact, the cursor goes square, even though I can do circles) which is at absolute zero and doesn't conduct heat and it can't be heated up itself.
Could be TPT going 'I have no idea what this is, but I'll make it as inert as possible.'
Edit: Hey, check it out in fire display, has a cool effect when you have lava on it. Also acid eats it.