Alveradok wrote
You can mine the 'stationary' ones by hitting them twice with a lightning bolt.
Or with a meteoric steel (or harder) pick axe. Unfortunately this will only net you blue and purple stones if you use meteoric steel, as meteoric steel has exactly the same hardness as green light crystal -- thus it can't dig through it. Fortunately however as far as light sources are concerned the three colors are functionally identical; they only vary in the color of the light produced.