How about a solid that stays the heat you heat it too only with heat or cool(not fire or LN2 etc).It also conducts heat. Just two things you could use it for are making yeast rise, and cooling steam without freezing it. There are probably more. Thanks. BTW: Do not say insulator, it does not conduct heat. And there is another topic kind of like this but it is fridge/oven and it is different. And also It has to be Invincible.
There are dozens of ways to make this in Powder Toy.
All you have to do is find a combination of elements that have convenient starting temperatures and line them up with clone and void so that any touching elements get the average temperature.