There are currently 4 different Shards in Portal Knights, each used when combined into Portal Stones make up and turn into the various portals to the various Islands. The shards are dropped from either killing Creatures or mining Resources throughout your travels. Depending on the higher level Islands you're visiting, the higher the level shard you'll find to drop.
List Of Shards
Each of the different color shards produce a portal stone of the same color when combined, as you can see below. To open a gate to a new world, you will need 6 portal stones of the same color as the gate.