Don't stay hot for long
I bought these to give my girlfriend massages, and they're okay. The rocks are smooth and a good shape for massages, but they don't stay hot for long. After about 20 seconds the rocks transfer most of their heat to her skin, and after a minute they are just barely warm. If you have a lot of these rocks and a large space to heat them up, you could make it work, but with only 4, they cool off too quickly and take too long to warm back up to provide continuous heat.
Rating: 3 stars