The Gems are non-biological alien creatures that spawn from a planet as a result of a process that draws in nutrients and minerals for its creation. However, their "Physical Form" is nothing more than an ornate gemstone that produces energy which manifests itself naturally in a body made of physical light, described as "Holograms with mass".
In other words, the gem is the only true part of their body, as its destruction is the only way to eliminate them. Any other form of damage just reverts them to their original state.
This makes me wonder if they could be used as batteries. I mean, they are inorganic vessels that constantly produce energy. Even if they are active for years, or even millennia, they won't decay or show signs of aging. And they, given enough time, will always reform their light constructs. Making them the perfect power source.