They can not be mined, they are a drop in Strat and Scholomance.
Blizzard needs to take another look at this item it is needed for the lvl 60 Pally horse, in the day, when you raided these lvl 60 Dungeons consistently, even though this is a rare drop, everyone eventually got one, in THIS DAY , at lvl 58 everyone heads to BC and none of these are ever seen. The old lvl 60 Dungeons just don't get any action, at least not on my server.
My guess is that they'll cost you a decent penny due to the shortage. The shortage being due to many people skipping the instances in which the diamond drops, and many others have moved on to the Outlands and will never run them again.
Unless you get someone who just doesn't know their use and value, and posts it for like 1G. =)