Gems are a form of artificially-created life where their bodies are projections generated by their gemstones, like a hologram with mass. They're extraordinarily resilient, but when seriously hurt, they will release their physical form and "poof" back to just their gem, where they can efficiently put all their energy into regenerating themselves. With this regeneration, they can also change their appearance from something as simple as growing their hair slightly or changing the color of an item of "clothing", or can change their form entirely. They can also change their "clothes" without the need to be poofed, mostly for cosmetic of battle purposes. As long as the stone isn't damaged when poofed, they can regenerate just fine, but if its damaged or cracked, their physical form can malfunction, causing parts of their to change size/shape at random and even effect their vocal patterns. If the gemstone is shattered completely, the gem is considered dead.
Gems are typically created from devices called Incubators, large devices that drain all of the energy it can (thankfully, an Incubator only drains the energy in a small radius, so there's no chance of large-scale damage to an ecosystem), causing the gem to literally emerge from the crust. The gem already emerges in its preferred shape, but there is the occasion that a gem will stay inside of the ground for too long and be slightly deformed, or "overcooked" as they call it, and there's even the odd occasion that some will just come out not quite right. All gems also possess a weapon of some kind, either being a physical melee weapon that they can generate from their gemstone like Pearl's spear, or an energy-like attack like Yellow Diamond's electric-like powers.
Gems are also capable of a technique called "Fusion", where two or more Gems combine into one. This is a very difficult process, however, with all participating members needing to be in perfect synchronization physically, mentally and emotionally, making it extremely hard for Gems who generally don't get along, causing them to separate. The new forms have extra body parts and the combined heights and strengths of the users. They even have the ability to create new combo moves from the participants' own move sets or can combine their physical weapons. Personality-wise, fusions do display traits similar to their fusies, but in any case, they're considered their own person with their own thoughts and feelings, but there are still some fusions that are just several Gems talking through each other.