I have seen many, marble,jasper,turquoise,tigers eye,Etc. and yes they do chip and break?
That's what I figured. Are there any advantages and do they work well as long as you baby them?
I don’t know about any advantages, but I’m guessing grain structure plays a role in cracking? I’m just going off what’s come across my bench over the years, although I’ve used a small river rock for a temporary bridge a few times? I’m not sure about the lifespan, I’m guessing it depends on how hard it is and how much it gets played? I think Randy Bretz might have used stone on his bridges before, you might ask him?
I tried soapstone, pipestone, glass and a couple of different cherts. they all had the same problems, they don't like the wrapped strings. However you can get around most of the cracking problem by making a wood bridge the same size as the stone bridge. And using it to get the exact placement for your bridge. A though body neck is needed with good support under the bridge area. If you planned on a true hollow body the cupping of the sound board may crack the bridge. I prefer bone, aluminium, brass water buffalo horn and antler.
I'm a bit a a crystal nut , and have collected crystal skulls and points etc .. for years , so of course , I have wanted to research the attributes of using crystal for guitar parts . Making a long story short , It's a myth that harder is better in a bridge , and many factors come into play , including hardness, elasticity, absorption, brittleness, etc etc . findings are . that jade , seems to be a favorite , and is close to passing the test on some points . another popular suggestion was corian (the man-made counter top stuff) , it also comes close to passing . Many compare it to bone . but not all agree it is better .
BUT- none really match up with traditional bridge material
Conclusion = Cool for a cool look . but most likely will sacrifices sound and practicability.
But take into account less strings , less tension , nylon . uke . diddly . etc ..
That being said, I have had a jade bridge on a diddly bow for about 10 years now . and all is fine and sounds good , but it is on a plastic (cyrstal cut ) kleenex holder body and acrylic neck .
So who knows ? . try it on something . it may work for your particular plan and set up.