Dude, when you cast molten metal into sand, you create glass micro balls. They are just starting from there. Sand is glass, heated to molten temperature and fused into a crystalline structure.
What is commonly understood as sand, has not much to do with little glass balls, even though here we have glass balls. A very special type of sand you can say, but not common "sand". But I feel no need to go more into pedantery here ... if this is sand to you, then I am fine with it.