The Dark Gems are evil jewels radiating with dark magical energy and they play a major plot element in Spyro: A Hero's Tail. These dangerous minerals represent part of the dark side by negatively affecting the game. Potent little orbs of condensed malice, the Dark Gems are capable of sucking out all of the light and goodness of the world, contorting the Dragon Realms into a putrid, scary den just fit for the future reign of the former Dragon Elder known as Red, who also placed such evil gemstones on top of his Sceptre to harness his powers.
Once planted, the Dark Gems drain the energy of the surrounding area, simply removing some environmental hazards or blockages caused by the life-draining power of the gems, will help you move on to previously unreachable areas.
The only way that the Dark Gems can be destroyed is by doing a Horn Dive, which Spyro learns by talking to the first Dragon Elder named Tomas within the Dragon Village. The Dark Gems are very important to the game, since it is necessary to destroy all 10 Dark Gems in each world to fight the boss for that world and advance to the next one. In this process, you can destroy a total of 40 Dark Gems in the game.