Crystal shards are crafting-materials that cannot be placed like blocks and cannot be put into your quickbar.
You can obtain crystal shards as loot or pet-harvest from Pigsies or sometimes from other creatures too, like from Chizzards or Night Chizzards. You can also process 1 block of Tourmaline each into 2 Crystal shards, but mind that Tourmaline is a rather rare find on the Stalactite layer and Lava layer, even though you can loot or pet-harvest it from Warmworms too.
Crystal shards are used in potions and for explosive, like (as of R25 in November 2015):