when you trade pkmn that evolve by trading with item is the item consumed because I trade a onix with metal coat to myself (i have 2 dsi XL) and then the metal coat disapeard
In most cases, the item will not be consumed(berries,evolutionary stones, tm's ect) but your case is special. Some Pokemon need to be traded while holding a specific item to evolve, like Onix->Steelix is with the Metal Coat, or Rhydon->Rhyperior is with the Protector. A full list of these Pokemon can be found here- http://pokemondb.net/evolution/trade
And marzipan is wrong, the Pokemon will not evolve if it is traded holding it's respective stone needed to evolve. The stone must be used on the Pokemon in-game.
Hope this helps!
Yes; the item is used up, just like a Fire Stone is used up when it's given to a Pokemon that needs it to evolve.