The Gem Sentry is a hostile mob encountered only in The Snare. It is a white, crystalline mob consisting of a tower and a pivoting orb "head" with a single green eye. It fires a beam from its eye, which creates a small explosion wherever it hits (not damaging blocks) that will deal 10 damage to the player and can possible knock them off ledges and into the void.
The Gem Sentry has 10 health, and is vulnerable to Soul Crystal weapons. If killed, it will drop quartz.
Thanks to its ranged attack, it presents a threat to any player trying to fly through the Snare, through an elytra or other means, but is less dangerous to players travelling on foot.
The Gem Sentry cannot move. It will float if the block below it is mined. It cannot drown and is invulnerable to fire damage, as well as poison and wither potion effects.