My wife loves rocks, and she wanted to play a druid that specializes in them. I was thrilled to see that not only was there a crystal druid, but it was so well done.
The druid is great because it gives the player a ton of more options without feeling overwhelming or too fiddly. There is a solid amount of different crystal effects, and the fact that each one has two modes, and that each one scales, adds that much more to the flexibility.
I suspect the decision to make using the crystal a bonus action was made so that the druid could create one and use one in the same round, but I like the design choice of using a bonus action to activate the crystals for a couple of other reasons. First, it gives the druid and their companions a new way to use their bonus actions, which is great since many character builds don't utilize them to their fullest extent. Second, it prevents certain types of spamming, since all PC's only get one bonus action per turn, no exceptions. I could imagine a different version of this build where the druid hands a pile of crystals to an action-surging fighter who throws them all in one turn. The bonus action idea was great.
The crystal golem is great, and it was a nice touch to make it have different elemental abilities like the small crystals.
The power level is very strong compared to the circle of the land druid, especially if your campaign doesn't have a lot of multi-encounter, short resting dungeons. At first I was going to give the crystal druid 4.5 stars, because I felt that the life crystal's thrown effect was an overpowered source of out of combat healing. But upon comparing it to spells like Prayer of Healing or Aura of Vitality, it heals at a similar rate. Plus the instantanous healing doesn't compare to a cleric's level of raw hp restored, range of healing, and number of targets, so it doesn't step on a cleric's toes there either.
If I had to make a suggestion, it would be to give the crystal druid the ability to talk to rocks, or create a spell for them that allows them to speak with rocks (like Speak with Animals or Speak with Plants). That was the main thing that my wife wanted, but I will just give her that ability at level 2 when she chooses the circle.
[5 of 5 Stars!]