Focus on textures rather than color: mix a matte stone (shungite) with a shiny stone (obsidian) and a stone with patterns (Dalmatian jasper) to avoid a look that’s too uniform.

Yes,onyx andblack obsidian are particularly popular for unisex and men's jewelry, especially in the form of round beads paired with stainless steel.

Gently wiping them with a soft cloth is sufficient for most of these stones. Avoid chemicals that can dull the polish.
It’s also best to avoid ultrasonic cleaners and to remove your jewelry before showering or swimming.