You can trade them for stones, depending on the colour. To do this, go to Hunter's house on route 124 just west of Mossdeep. If you have a shard in your bag, he will ask to trade it.
Red shard = fire stone
Yellow shard = thunder stone
Green shard = leaf stone
Blue shard = water stone
everywhere
Catch Clamperl, Chinchou and Relicanth most of these Pokemon hold shards
They can't be bought. You can find them underwater, or attached to wild pokemon. Chinchou have yellow shards, Clamperl have blue shards, Relicanth have green shards and Corsola have red shards.
Use the move dive to go underwater to find shards in the seaweed.
they come from the clamperl
Blue shards are found underneath the water while diving.
Green shards are found underneath the water (or in the sand on islands) while diving.
Red shards are found underwater.
You can trade them for evolution stones eg. yellow shard = thunderstone
you give a guy a certain amount of shards and he will trade it to u
The shards are on the bottom of the seafloor along Route 124. Normally there will be on a spot that's darker than usual. Some of the shards are also in spaces between the grass. Some of the shards are also in the pokeballs you find by exploring the deep-sea troughs and resurfacing along Route 124.
Yellow Shards and the other color shards are used for trading with NPCs for items or moves. For example in Pokemon Ruby, Sapphire and Emerald they are the main source of evolution stones.