Not in the wild; you must trade it from FR/LG/E.You can obtain Deoxys in Pokemon Diamond by:CheatingTradingEvent
You will need beat the elite four in Pokemon Indigo in order to obtain Deoxys. After beating the elite four Deoxys will randomly spawn.
Deoxys is on Birth Island, you can get there once you obtain the Aurora Ticket from the events.
You have to have a cheating device like a game shark, or action replay to be able to obtain Deoxys in Soul Silver
The only way to obtain a Deoxys is to migrate it from Pal Park.
Trade.
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
Migrate it.
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darkrai and arcus
a REAL diamond? lol in a cave, if the Pokemon one, download it or buy it
The only way to obtain Articuno in Pokemon Diamond is to trade it from GBA via Pal Park.