Tips for use:

The semi-procedural crystals are quite high poly but this can be mitigated by using smaller values for the "Final Merge Distance" value in the geometry node sidebar. For the renders shown I used the value of 0.25cm; increasing this results in lower poly and less detailed crystals. You can also adjust the "Crystal Density" to produce a less dense mesh and lower polycount.  Once you have a crystal seed you like, another option to lower poly count would be to convert this to a mesh by applying the geometry nodes group and then use a remesh modifier to reduce the polycount (apply, then re-assign the crystal material so that it shades properly).