Moonstone
Orthoclase (potassium feldspar KAlSi3O8) is transparent and colorless in its pure form. This is called adularia.
As for the moonstone, characterized by its white or bluish shine (called adularescence), it is a mixture of orthoclase and albite, with a layered structure that generates this effect.
If these layers are not very thick, the light reflecting off them produces interference effects that give rise to the bluish glow. On the contrary, if these layers are very thick, the white glow would appear.
To get the most out of this effect, the cabochon should be cut in such a way that the base is parallel to the plane of the layers.