In terms of their usefulness, Rumblestones have found themselves widely used by the Dwarves. Dwarves have a great fear and respect for the rumblestones due to the fact they can cause cave ins, but also a greed for them as well do to how lucrative even a small amount can be. Large amounts of Rumblestone can be crushed into sand and gravel, left out in long trays, and after they've collected some energy they can be used as a conveyor belt for dwarf smithies- metal objects placed on top of the sand are shook and slowly moved down the tract to the next station.
While many purposes of Rumblestone are industrial or for magical research, even more primitive cultures have found powerful uses for the material. Rumblestones can be found occasionally in a large enough stone that simply tying them to a stick makes a powerful makeshift magic weapon, and in fact many primitive cultures have these as their most powerful magical weapons.
Stats- 1d6+1 for a standard sized stone
Glowing stones, knapped and tied into a primitive club weapon. More advanced cultures can use a metal hand and shaft instead, but the damage and stats remain the same. Rumblestones are like normal, heavy rocks except very hard compared to most stone. Additionally, they are infused with Earth magic and let someone using one treat their caster level as 1 higher when casting an Earth magic spell. This weapon has a very weak ego, and those without the spirit to master it are only beset by a 1 in 6 chance to have it 'quake' on them, shaking their arm and throwing off their aim with -2 to hit. This only happens once per combat at maximum.
The weapon deals 1d6+1 damage on a hit, which counts as magic. It can damage anything made of stone, such as rock or marble golems, even though it may technically be an inferior material. When using this weapon to destroy shields or sunder armor, enemies make their saves at -1. Additionally, this damage can rumble apart small stacks of objects like cairn stones or piles of books or bone, the shockwaves traveling through the mass to shake it apart.