I am cooking a pressed apple terrine (a derived version of this one from Raymond Blanc) where I need a weight to put on the apples during cooking. Problem is, the only weight fitting I have that fits the terrine is my whetstone. I wanted to know if the heat could damage it, or if it could be hazardous for my dessert ?
It's probably not a good idea.
If you know your whetstone is natural (Wikipedia) then it should be all right with a food-safe material in between (like baking parchment). Water-stones may well crack unless very well dried, so put the stone in a cold oven and turn on to about 80°C for a couple of hours, then allow to cool before using as a weight.
Most modern whetstones are resin-bonded ceramic and the resin probably can't handle that much heat without problems. The heat could damage the stone, release unpleasant smells from the resin into the food, or even release toxins.