On Route 124.....
yellow shard and red shard and green shard
You get the red shard from the red garados if you fail to catch it at the lakeof rage.
you have to get a blue shard and then give it to shard dude (i dont know his name) BLUE SHARD=WATERSTONE RED SHARD=FIRESTONE YELLOW SHARD=THUNDERSTONE GREEN SHARD=GRASSSTONE
There is a hunter west of Mossdeep city that will take special shards like the red shard trade your shard for a evolutionary stone so the red shard will give you a fire stone.
thats not a shard trader his a diver and his at route 126.
In the Underground.
You don't need a Red Shard for Dive.
In Platinum, the Red Shard and the other Shards are very valuable if you want to use 1 of the 3 Move Tutors there because you have to give them certain Shards for certain moves and the Red Shard is 1 of those many Shards so that would be a reason of why someone would want to get a Red Shard.
Red Shard Blue Shard Yellow Shard Green Shard Violet City and Fuchsia City
I found a red shard after using CUT on a few trees, it was lying around as an item. Was near Rustburo i think.
The red shard can be given to the guy next to the Pokemon center for a cheri berry,a pecha berry and a leppa berry.
Red shards are found underwater.