Bag of Expanding Stones

This small, unassuming bag is made of durable, dark leather and is roughly the size of a fist. When you open it, you see that it is filled with an uncountable number of small stones, each exactly 3 cubic centimeters. As you reach in to grab one, you notice that they seem to shift and move of their own accord. The bag contains 10d6 stones.

When thrown as a bonus action, the stones’ size can be increased to whatever size the thrower wants it to become, with the density decreasing as the molecules of the rocks spread out. The weight of the rocks does not increase. At exactly 16.53 cubic feet, the stones become as dense as air. Any larger, and they begin to fly upwards. 

After one hour since the rock expanded, the DM must roll a d100. On a 1-50, the rock decreases back to 3 cubic centimeters. On a 51-80, the rock becomes its new size permanently, with its weight increasing to match its size. On an 81-100, it increases in size uncontrollably, and any creature within 20 feet must make a DC 13 dexterity saving throw, taking 55 (10d10) bludgeoning damage on a failed save, or half damage on a success.