Board Thread:Dragon City Gameplay Questions & Answers/@comment-25199624-20140730005129/@comment-24568135-20140730180542

Use the Dragonarium and Kindergarten.

I have gotten Crystal with:

Pure Terra level 4 and Pure Ice level 4 54:00 Crystal

Pure Electric Level 7 and Pure Metal level 7 54:00 Crystal

Pure Metal level 4 and Pure Ice level 7 54:00 Crystal

There doesn't seem to be a best way.