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If you are building a sauna that uses hot rocks, as in a Finnish sauna, you'll need to select stones for your heater. You may think that any stones will do, but because of the stress the stones are under by constantly being heated and cooled, regular stones will crack quickly. Therefore, select stones with care.
Good qualities of a sauna stone are:
• Heat resistance
• Does not smell bad when wet
• Good heat capacity
• Rough surface to more quickly release steam
Good minerals to use for stones in the sauna are peridotite and olivine. Most sauna manufacturers will sell the stones, or you can gather them yourself and put them through a stress test. To test stones, heat them for about an hour over a fire and then immerse them in a bucket of cold water. If cracks appear or a grinding sound emits, discard the stone.