I recently acquired a mala (buddhist rosary) made out of lava rocks. Here's a picture of one like mine. I'm using it to invoke the fires deep inside the earth. I haven't done it enough yet to really know if it's working or not, but here's the invocation I've been using. I recite it 108 times, using the beads to keep count. (Which, after all, is what mala/rosaries are for.) I've been wearing it much of the time.
Fire of the earth, warm my bones.
Fire of the mountains, warm my heart.
Volcano stones, remind me.
UPDATE: It's working.