I set up twenty lightwells. Put in ten standard aquamarine. Put in ten Size 1 crystals. Before I could even finish putting in all ten rock crystals, one of the rock crystals had broken, and a second broke within seconds of me finishing placing the last one.

By the time I was down to the last rock crystal, I still had seven aquamarine remaining.

Rock crystals do not provide more liquid starlight than aquamarines. A large, well shaped one might, but the book does not say the crystal must be large and well shaped.