If you mean the Thunder stone, you go to the New Mauville Power Plant.
find it using dive
dive any where possible between mosdeep and lilycove and get a yellow shard go to the guy who lives on an island and he will trade a thunder stone for your yellow shard
If you mean the Thunder Stone, go to the New Mauville Power Plant.
You first need a yellow shard these can be found using Dive near the Hunter's house, Second give the yellow shard to the hunter he will trade you a thunder stone for the shard. Another way to get a thunder stone is by trading a Pokemon from firered or leafgreen that is holding one to ruby, sapphire or emerald.
You don't need a Red Shard for Dive.
After you have defeated the hideout at Lilycove city and head for Mossdeep city you will see some patches of dark water which you can dive in after you have defeated the gym at Mossdeep city you will be able to dive. If you go to the outskirts of the city you will find a house . In that house a man says that if you find him a shard he will trade with you. The dark patches of water you saw before are the places you must look for shards. when you are underwater look for marks that will have items in them. Once you have a shard it could be a green shard, red shard, blue shard or yellow shard. A green shard will get you a leaf stone, a red shard will get you a fire stone, a blue shard will get you a water stone and a yellow shard will get you a thunder stone.
you can find it on route 124 I've had a vulpix and to make it evolve i had find the red shard and i had to give it to hunter in mossdep city to get the fire stone
you can only get it by buying it at lilicove or change it for a blue shard
Underwater
find a blue shard using Dive, and trade it to the guy by Mossdeep, or catch some clampearl, they hold blueshards too.
to get a red shard in emerald, you have to go to route 124. Hope this helped You have to dive underwater using hm dive and find them hidden in pale couloured stones, such as those you see in caves
give the shard hunter a yellow shard found under water near mossdeepcity and sootoplis