At its heart, a soul gem is nothing more than a crystalline binding agent. They are common in species that are connected to natural plants, minerals, etc. They are also commonly used as a means to focus and amplify Spark power to interact with animism.
Each individual species will have evolved a niche set of powers and functions for the soul gem.
Regardless of any other function, the soul gem transfers 1 Essence from the body of the character into the Soul Gem. It does not decrease the Essence total of the character, it just alters how it stored.
Powers can be automatic, or sometimes may require or consume for energy a part of the characters Essence total.
Stabilizes in Death: If the character drops to 0 HP, they automatically stabilize. It will not stave off Death Saves from any subsequent drops.
Bonus to Death Save: The character get a +1 (weak) or +2 (strong) bonus to their Death Saves.
Flora Protection: Immunity to secondary effects (non-damage) of cantrips associated with plants
Essence Amplification: +1 Essence
Poisson Resistance: Resistance to poison damage type
Flora Obfuscation: 1 Essence triggers the Pass Without Trace spell as a Bonus Action.
Flora Communication:: 2 Essence triggers the Speak with Plants spell as a Bonus Action.