We are talking about phenomenal stones here, and part of what
distinguishes moonstone from labradorite and spectrolite (which is
basically black-base labradorite) is the phenomenon displayed.
Moonstones display a phenomenon known as adularescence. Adularescence
is that subtle, billowy, silvery sheen which plays across the
surface of a moonstone. I know that this is in part a circular
definition, but I didn’t invent the terms. Labradorescence is the play
of color which flashes from labradorite (bet you didn’t see that one
coming.) Yes, labradorite and moonstone are both feldspars, but they
are characterized by distinct phenomena. My thought is that if
feldspar displays a play of color, it is a labradorite and not a
moonstone.
Lee Einer