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Feldspar: a rock-forming tecto-silicate mineral that crystalizes from magma and makes up about 60% of Earth's crust. Now, obviously not every feldspar is a gemstone. You can find this common mineral in building materials, glass, and even non-dairy creamer. Feldspars can be rich in various elements, and when you combine some types of Feldspars with others and layer them, this is when you get fun optical illusions and gemstones.
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Moonstones (and some sunstones and amazonites) have a property called "adularescence." Since moonstones are made of up many fine layers of feldspar, light reflects between these fine layers and gives moonstone its milky luster, often called "schiller."
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Now let's talk about labradorites (and interestingly, blue/rainbow moonstones and some sunstones are actually labradorites). Instead of light reflected between layers of feldspar (as in schiller), various layers of differing feldspar *interfere* with light waves and makes them scatter randomly. This interference causes the various colors of sheen, which change at every angle.
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These gemstone feldspars have been regarded as magical for centuries in MANY cultures around the world. For example, moonstone & sunstone were regarded by the Ancient Greeks as coming FROM the celestial bodies themselves. Any ritual involving Moon/Sun deities would involve these stones.
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Lovers would exchange moonstones. Since the moon renewed every month, so would their love. Magicians, who believed their power came from the Moon, used moonstones.
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In the 1920's, these stones became popular again with the Art Nouveau movement.