I also just got the diamond dragon as a reward for mining (for land), and the reward dragon was also nowhere to be found in my case. I went from completely out of my mind excited to completely bummed out. Just sent a message to support@teamlava.com... I hope they can help.
Hold on!!! How can u win diamond dragon from mine o.O
So they say but no one can find it!
It's a pop up after the last update ~ along with some gold rewards. Did you get the march 7th update yet? Also, mines rewards are doubled for the weekend too!