Hi, I answered on your other threads, but I will post here in case anyone else is wondering

It is the same as stacking with any bangle, the spikes do not pose any additional risk since they are pliable, but the metal itself could scratch a softer stone by rubbing over it, such as MOP, GMOP, turquoise, Malachite. VCA guilloche could also be scratched. Hammered gold should be fine. The harder stones like onyx, blue agate, chalcedony, carnelian etc will be fine. I stack an onyx bracelet with cartier bangles, including the Clash at times with no issues. I have blue agate also, and it will not scratch easily at all, the only concern I have heard is that in rare cases it can crack if dropped or hit on a hard surface.