Poor granite. This species suffered a near 100% failure rate. I’ve been told by a geology professor that water can infiltrate the interstitial spaces between the relatively-large quartz, feldspar and mica crystals and expand the rock when heated. To it’s credit, granite never sent shrapnel flying when it cracked.
Quartzite fared a little better, being smaller-grained and more homogenous throughout it’s structure. About 50% of these survived. I will note that when quartzite cracked, it sometimes exploded (but shards never traveled more than a foot or so away from the wooden bowl.
And the winner is . . . .
Basalt out-performed the other species of rock by far. Only about 5% of these cracked with use. A few years later, I still use these particular pieces of basalt. They show no signs of giving out. Incidentally, one can purchase smooth basalt cobbles in fancy home/yard furnishing stores.
