This is probably the most incredible science DIY I have come across in a while. Check out this awesome video by Youtube channel, HouseholdHacker. All you need is milk and white vinegar to make your own hard-as-rock "stone" that can be shaped into just about any object you desire.
Take 1 warm cup of milk and mix with 1 tablespoon of white vinegar. Basically, for every cup of milk you use, add 1 tablespoon of vinegar and you can make a much larger batch. Mix it together and strain out the curds that form. Once you dry it you can mold it into any shape you desire, but let it dry for a couple days first!