unstable transmutation

You choose a small or medium sized stone that you can see that fits within a 5-foot cube around the caster, it takes half a turn until its armed and starts glowing with a slight blue glow

Any creature in a 3-foot cube around it must make a Dexterity saving throw. on a failed save the stone explodes dealing 1d6 fire damage to anyone in a 5-foot cube around, and gets disadvantage on their next roll

if there are 3 armed stones in a 5-foot cube they start glowing red and in 1 turn explodes dealing 2d6 fire damage to any creature in a 10-foot cube around