Feldspar
The feldspars are a group of minerals that belong to the silicate family. Feldspars can be divided into two types: plagioclase feldspars and alkali feldspars. They have a glassy luster and a hardness of 6-6.5. The chemical composition of all plagioclase feldspars is between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈). Alkali feldspars vary in chemical composition between K-feldspar (KAlSi₃O₈) and albite (NaAlSi₃O₈). These “in between” compositions represent a homogeneous mixture. Although the solution is stable at high temperatures, at lower temperatures ions can only limitedly substitute for one another. As the mineral cools the ions (Na⁺, Ca⁺², or K⁺) may segregate, resulting in layers of alternating compositions (lamellae). These lamellae can range from submicroscopic to visible in a hand sample. This is exciting because these lamellae can produce some very interesting effects.

Labradorite
This mineral is a grey to black plagioclase feldspar that contains 50-70% anorthite. The lamellae present in this mineral are very fine, with a thickness approximately equal to the wavelength of visible light. This spacing may act as a diffraction grating, both scattering the light into spectra and causing interference of light reflected by layers of lamellae with different indices of refraction. These optic phenomena combine to create an effect known as labradorescence- a beautiful sheen of blue, green, gold or orange.

Amazonite
Amazonite is microcline, a variety of alkali feldspar. What distinguishes it from other microcline feldspar is a striking blue-green color due to trace amounts of lead. Microcline commonly develops lamellae visible to the naked eye- slow cooling is what allows these large albite intergrowths to occur. The albite lamellae appear as white braided, net-like or irregular patterns on the green stone.

Moonstone
Andularia (a variety of alkali feldspar) that exhibits iridescence is known as moonstone. Moonstone is colorless, white, grey or peach, but it glows with a blue or white sheen below the surface.The iridescence, referred to as andularescence, is due to a submicroscopic lamellar structure that scatters light and creates interference. As a consequence, optic phenomena similar to those present in labradorite are produced. Other varieties of feldspar that exhibit adularescence may also be referred to as moonstone.

Sunstone
Sunstone is a variety of plagioclase feldspar that is 10-30% anorthite. Plate-like inclusions of hematite (an iron oxide) within the feldspar lend it a golden to red-orange color and a coppery shimmer. It is similar in appearance to aventurine and is sometimes referred to as aventurine feldspar.

References
Klein, Cornelis 2002. Mineral Science. John Wiley and Sons, Inc., New York.
Nesse, W. D. 2000. Introduction to Mineralogy. Oxford University Press, New York.