That is a good question! I had the impression it was fairly stable, I don't know by what process amorphous can turn into crystalline... silica range : use of silica, range of silica, manufacturing process here it says that non-crystalline ones can all be converted into crystalline forms when heated to high enough temperatures, and from reading stuff elsewhere it looks like in its anhydrous form it can trap a fair bit of water, but I have not seen anywhere that this could convert it to its crystalline form. I don't know how hydrated the silica in the HD powder already is, though.