Paul, looks very similar to what in Spain we call 'Cuencos de Calabaza', Google told me that these are gourd bowls in english. Found a couple of links on how to make them.
http://www.instructables.com/id/How-to-clean-a-hardshell-gourd/?ALLSTEPS
In Spanish (If you need help with the captions let me know):
https://www.youtube.com/watch?v=sZ3qBNyjSdU
Also I found this about Jicara. That sounds pretty similar to jacama:
https://www.youtube.com/watch?v=ioVS-ZF9A34
I know that you say these are stone, but looking at your photo (there are some 'ribs' and what it looks like the base of the stem) I would dare to say that they are dried calabaza or jicara.
Hope it helps.