The Crystal Shards are objects that appear in Sonic Boom: Shattered Crystal. They are fragments of the Lost Crystal of Power which the Ancients shattered into several Crystal Shards 5.000 years ago and then hid away to keep them out of the grasp of Lyric the Last Ancient.
The Crystal Shards are hand-sized purple (light blue in the in-game comic) shards with a rough surface. Each containing a fragment of their aggregated form's power, the Crystal Shards can be harnessed to warp reality to some extent, like when the Ancients used them to imprison Lyric.
While the shards are powerful enough for the Ancients to deem them dangerous, they do not seem match up to the Lost Crystal, even when used together, since the Ancients were only able to imprison Lyric temporarily when using all of them.
In Sonic Boom: Shattered Crystal, about 5,000 years ago, when the power-mad Lyric the Last Ancient was on the verge of obtaining the Lost Crystal of Power which would give him omnipotence, the Ancients opted to destroy the crystal, shattering it into forty Crystal Shards which they used on Lyric to imprison him. However, the Ancients knew it was only a matter of time before Lyric would return, so they hid the Crystal Shards away across several islands to prevent the Lost Crystal from being reassembled.
Lyric eventually escaped in the present day and started searching for the Crystal Shards. Meanwhile, Amy Rose had discovered a secret ancient language and manuscript detailing the history and whereabouts of the Lost Crystal of Power's fragments. As such, Lyric kidnapped Amy to probe her mind for information on the Crystal Shards, but Team Sonic put a stop to his plan. During their journey, the team also found all the Crystal Shards and reassembled the Lost Crystal.
One of the side quests in Sonic Boom: Shattered Crystal is to collect all forty Crystal Shards which are hidden in the adventure levels. Each of the eight adventure levels in the game holds five Crystal Shards inside them, each of which can be collected by opening a spherical silver and red container with a Crystal Shard inside it. These containers break when touched and can be targeted with the Homing Attack. Some of these containers are well hidden while others can be found on the main route through the adventure levels. Notably, the second Blueprint upgrade, Map Accelerator 2.0, displays the location of individual Crystal Shards on the Adventure Map, making it easier to find them in the adventure levels.
Collecting all five shards in an adventure level will unlock a Sonic Badge. After obtaining them, the player can bring the shard they have obtained to Sticks' Burrow to reassemble the Lost Crystal. Additionally, the shards used for the restoration will fill in various spots on the crystal, affecting the way the top screen looks during loading times. After all forty Crystal Shards have been combined, the Lost Crystal will be rebuilt and the player will receive a purple Token as a reward to unlock the 61st toy in the game with,