Board Thread:Dragon City Gameplay Questions & Answers/@comment-2600:1700:F4E0:8240:ADB6:6DF6:49A3:E2E4-20181124085100/@comment-2601:44:8902:203E:5587:25DC:74E2:18BB-20181125185720

If it’s an a plant crystal, then it is a bug, but if it’s a terra crystal it’s because they have to be the same element (crystal and habitat)