The Dark Shards are minerals comprised of dark magical energy that are actually the smaller versions of the Dark Gems themselves and objects featured in Spyro: A Hero's Tail.
Instead of helping the dragon Spyro get to new places, Dark Shards help him gain more Gems, which can be spent at Moneybags' shop. Dark Shards can also be destroyed by charging, unlike the Dark Gems which require the Horn Dive in order to be destroyed.
The Dark Shards are also the targets in the levels where the Professor's nephew Blink is, the object being to find and eliminate them all in the level. In Blink's levels, they can also be only destroyed by bombs.