I tried a lava shell massage a while ago at the Cheeca Lodge in Islamorada, the Florida Keys. I really enjoyed it. I liked it better than the traditional hot stone massages. The shells are filled with a mixture that start a chemical reaction that heat up the shells. There is sort a periodic popping sound as they continue to re-heat. Kind of like the sound the Pop Rocks candy makes. 
Anyway, they get really hot and without the inconvenience of sometimes having water dripped on you, or having the stones cool off too quickly.
And yet, I haven’t seen many places offering them. I found them recently at a new spa I was visiting, and wanted to try it again. I ended up waiting for a while to go back to my treatment room. A long while… the reception told me eventually that my room was being prepared… When I got back to the room, therapist told me that the shells were not heating up appropriately. I didn’t know if they were that finicky, or if they weren’t used often at this spa and the chemical agents had a shelf life. So, my therapist improvised and said there were a couple of lava shells that would heat sufficiently and that she would use some of the stones, and that she also used her hands to get into the tough places that needed work.
Therefore I had this combination of therapies. As I laid there under her ministration, completely relaxed, I realized that truly, the most effective method of eliminating the knots was the work of the therapist’s hands. Its not the stones, or the lave shells, but a gifted therapist that really makes the most difference which is why when you find a great therapist it makes all the difference in the world. Don’t get me wrong, I will still try the lava shells and hot stone therapies at times, but sometimes the basics can be better when it’s at the hands of a skilled therapist.