The only way to get a diamond is from beating dialga (diamond only) by capturing him at MT coronet.
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If I'm correct, the Gracidea flower cannot be obtained in Pokemon Diamond. Sorry! (Note: The Gracidea can be obtained in Pokemon Platinum.)
From Great Marsh after you have obtained National Pokédex.
no
Once HM02 is obtained from Veilstone City
TM24 (Thunderbolt) can be obtained at the Game Corner in Veilstone City, for 10000 coins.