If you want to make it so that when you click a diamond it doubles you need to make a "PlayerInteractEvent". Then you check if the clicked item is Material.DIAMOND and if so set the amount two 2/double the previous amount.
Ok I understood almost but how to set the amount 2/double the previous amount? Hi you're on online do you know how to set the amount 2/double the previous amount? EDIT by Moderator: merged posts, please use the edit button instead of double posting.
This is a forum; he will answer when he has the time. Also, he has no obligation to help you. Get the amount using ItemStack#getAmount(). Multiply the amount by 2. Set the new amount using ItemStack#setAmount(int). Note: You might want to max the amount by the material's maximum (usually 64, but it is 16 for ender pearls, for example).