Diamonds can be earned/won in a few ways. Buildings can ocassionally give diamonds. Genies (won in the Spire, chapter 3 and above offer from 50-200 diamonds if you are lucky) and Wishing Wells (may have been discontinued, haven't seen them as prizes in events for a long while offer 25? or was it 50?, again, if you are lucky.). In addition the Spire chests (ocassionally you may get from 15 to 125), selling portals back (at the end of each chapter sometimes the last quest for that chapter gives you 200). And finally, crafting has a 20% chance of 500 diamonds every other time you amass 100 vision vapor and unlock the Mystical Object. Vision Vapor is one of the things you earn when you craft an item. There may be other ways of which I'm not aware or have simply forgotten.
Just to let you know I craft like crazy and complete the spire every week. My diamond count has gone up as I've done so...but I've also bought diamonds in the past so it's difficult to say if I've won/earned enough in the game to have had the fun/'growth I've had without the small purchases. I'm pretty sure there are many, many who never purchase diamonds and manage.
AJ