Here what you should consider modding in for the damage handling and biology or something:
Blunt objects won't work against any body parts on gems, except directly on their gem. (Hammers and rocks won't do anything, except if they hit the actual gem)
Gem placement has to/should be random, gems could be on feet, fingers and maybe noses.
Gems do not need to eat, sleep, drink, or breath. They have no skeleton, organs, blood, skin, any kind of tissue or nerves, or muscle.
Any damaged/cutoffed part of a gem except the actual gem evaporates immediately
Limbs can get cut off, but if torso or head are pierced, the whole body dissipates.
The only type damage any body part of a gem except the actual gem is getting cut off or pierced.
Every body part on a gem can regenerate except the crystal/gem (you need to make the regeneration all at once if you can. You could do this by making regeneration a natural ability, and when used, regenerates very body part but the gem)
Gems also can spawn a weapon as part of their natural ability, and when thrown (after it hits) or dropped, it evaporates.
The weapon you spawn corresponds with your starting skill choice.
The body should be made out of a material that is ultra hard to smash, but less difficult to cut. The actual gem itself should be the opposite, easy to smash, but nigh impossible to cut.
The only [nervous] or permanent damage a gem could get is on their gem, which might limit how much they can regenerate.
Gems can jump extremely high, and have enormous physical strength compared to humans. They are about on the same level as a bronze collosus in terms of strength. Due to their immense hardness, they can take falls from cliffs, explosions, and being thrown at ludicrous speeds as long as the don't land on their gem.
If the actual gem is chipped or smashed, it may cause unconsciousness, blindness, loss of ability to regenerate, loss of ability to stand, loss of motor ability, etc.
You die when your gem is shattered.
Also, fusions and stuff. Basically you combine your abilities and skills, and gain another set of arms and eyes. If any gem in a fusion is chipped or shattered, de-fusion occurs.
Also shape shifting, they can turn into anything they want, however whatever hey turn in to is still made of the same material and abides by the same rules as the gem's original form.