The gemstone crab is a boss that moves between the three mines in the Tlati Rainforest every 10 minutes. It has no defensive stats and effectively infinite hitpoints, making it ideal for combat training, and can be fought in large groups. The only requirement to fight the crab is completion of Children of the Sun in order to access Varlamore.

The health bar of the monster doesn't show its actual health, but rather how much time players have left before needing to switch locations. The crab remains stationary for exactly 10 minutes, after which its Hitpoints drop to zero. At that point, it burrows underground and relocates, requiring players to enter a nearby cave to continue the fight. All three caves lead to the crab, wherever it happens to be.

Players will earn slightly less experience per hit than usual, at about 87.5% the usual experience (1 damage = 3.5 experience rather than 4). Unless at lower combat levels, the gemstone crab typically offers higher experience rates than other crabs because there is no downtime in between attacks and no experience gets wasted for overkill damage (doing more damage than the remaining Hitpoints).

Upon defeat, the first 16 players who dealt damage to the crab gain the opportunity to mine a piece of the crab's shell for three random unnoted gems.

Since you are not technically killing the boss, the boss does not count towards Crab slayer tasks.