unstable gemstone

You create a large gemstone of any color weighing 260lb in a spot within range not currently occupied by another object or creature. The gem has an ac of 10, 30 hp and resistance against piercing, slashing and fire damage. Upon hitting 0 hp the gem explodes dealing 2d12 force damage to all creatures in a 30 foot radius and all creatures large and smaller have to make a dexterity saving throw or be pushed 30 feet away from the crystal.