If you dont have a diamond/gold pickaxe you may still get diamond/gold by using iron pickaxe
Yes, it can destroy the block, but the diamond block will disappear and you will not get any diamonds. You need and Iron Pickaxe or better (Gold and Diamond) to mine Diamond, and receive the diamonds.
To get diamond, you need a pickaxe of iron, gold or diamond.
iron diamond gold and leather
You can use an iron, gold, or pre-existing diamond pickaxe to mine diamond ore or diamond blocks
You can mine redstone with an Iron, Gold or Diamond pickaxe.