How do I send diamonds to someone?

To send diamonds, you'll need to go to the exchange. Trades>Currency Exchange. It's better if you have them set up a trade for however much gold [if you're giving it to them, most people know to stay away from the 1 gold for ___diamonds trades] that way you don't have to worry about someone else going into the exchange at the right time and buying them.

/so yeah. Have them set up a trade of their gold for your diamonds and then go in and accept.

Also, make sure to set the Gold for Diamonds/Diamonds for gold part right or else you won't see each other's offers.