Crash Bandicoot 2 has 25 total collectible crystals. Each crystal is tied to one of the game's 25 main levels, and collecting it is normally how Crash clears that level and returns to the Warp Room.
The crystals are central to the plot. Dr. Neo Cortex tricks Crash into gathering them, claiming that an approaching planetary alignment could destroy Earth and that the crystals are needed to contain its energy. Cortex actually plans to use that power for the Cortex Vortex, a space station designed to brainwash the planet's population.
Players often confuse crystals with gems. Crash Bandicoot 2 contains 42 gems, which are earned through challenges such as breaking every box, finding alternate routes, completing secret areas, or meeting time-based requirements. Those gems eventually help Dr. Nitrus Brio destroy the Cortex Vortex. The crystal total is therefore 25, while the gem total is a separate completion goal.