"Blue rays" become visible in rock crystals only at a certain angle of view (i.e. at a certain angle of inclination of the crystal). But if they appear, then it is very impressive - in the form of a network of thin intersecting (usually at an angle of 60 °) rays, shining blue, like the thinnest rain streams illuminated by the sun. This phenomenon is due to the scattering of light by a system of short thin cracks oriented in the crystal parallel to the faces of a rhombohedron or (rarely) a basopycanoid and healed by the smallest gas-liquid bubbles. Two opinions have been expressed about the reasons for the appearance of such cracks: either they are of a contraction nature and are associated with the precipitation of impurities from the crystal lattice of quartz with decreasing temperature, at the transition, or they arise in connection with the formation of linear defects in the crystal - narrow hollow channels, diverging like a fan from solid inclusions in quartz. It is possible that both proposed mechanisms for the formation of "blue rays" are at work.
You can see stones with the effect of blue rays for sale and buy it in the Minerals of Russia online store.