It is a hold item that doubles the pokemon's attack power, but makes it confused. The item is used up, and dissapears.
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not unless you cheat and duplicate it
go to cerulian city and talk to a kid that's be hind the house where a man will ask you if you collect badges and when you talk to that kid his itemfinder will go off then go up to the bridge and surf down the water to where mewtwos cave was and when you see the kid again surf up next to him and turn to the west and go to the wall there should be one spot to your left and right filled with water then lookto your left and hit the "a" button and you will get the berserk gene
The Berserk Gene is in the water; use Surf to get to it and the Itemfinder to pinpoint exactly where it is. Then just press A when you're in front of it. Incidentally, it's meant to be the last traces of Mewtwo's presence since Cerulean Cave is inaccessible in that game.
its the silver apercorn in Pokemon silver and its the gold apercorn in Pokemon gold
No you can't catch Mewtwo in Pokemon silver, you must trade it from Pokemon yellow.