Crystal Shards can be collected from friends either by asking for them in the Workshop or by having friends send them to you directly as a gift (maximum once per day). (Formerly, they were also automatically gifted to your friends when when they visit your kingdom, you accept their help and then dismiss them. However, this is no longer the case.) They are also occasionally rewarded when you defeat a Beastie.
Crystal Shards are used to make Exploration Crystals in the Workshop.
Can be obtained from following Wall Posts
X has some 'splorin to do!
X is boldy growing where -- aw, never mind.