Feldspar is the name of a large group of rock-forming silicate minerals that make up over 50% of Earth’s crust. They are found in igneous, metamorphic, and sedimentary rocks in all parts of the world. Feldspar minerals have very similar structures, chemical compositions, and physical properties.
To appreciate the importance of feldspar as a rock-forming mineral, let’s consider its abundance in Earth’s crust. Most of Earth’s continental crust is made up of igneous rocks such as granite, diorite, and granodiorite. Feldspars are also important constituents of gabbro and basalt, which are the primary types of rock in Earth’s oceanic crust.
Several varieties of feldspar minerals are used as gemstones. Three of them, moonstone, sunstone, and labradorite, are known for their unique optical phenomena.
Moonstone is a gem material that consists of microscopically thin and alternating layers of feldspar minerals of different compositions. When light enters a polished gemstone and encounters these layers, it is scattered in many directions. This produces a glow within the gemstone known as adularescence. The glow appears to float slightly under the surface of the gem and moves as the source of illumination moves, as the angle of observation is changed, or as the gemstone is moved under the light. People enjoy this soft glow, especially when it has a striking color or the body color of the moonstone is pleasing. Orthoclase is the most common feldspar mineral to host the adularescent phenomena; however, it is also known in albite, oligoclase and labradorite.
Sunstone is a gem that contains tiny plate-shaped and highly reflective particles with a common orientation. When light enters the gemstone, it strikes these particles and they reflect it with a glittery flash known as aventurescence. The particles might be tiny flakes of copper, hematite, mica or other reflective mineral. Labradorite and oligoclase are the feldspar minerals that most often have aventurescence. In some localities colored but non-aventurescent labradorite is also called “sunstone,” but aventurescent material is usually mined nearby.
Labradorite is sometimes intergrown in microscopic layers with albite or other plagioclase minerals. When light enters a polished gem and strikes these layers at just the right angle, the light is scattered, its wavelengths are modified, and an iridescent reflection is produced. This can result in spectacular flashes of strong iridescent colors in electric blue, green, yellow, orange and pink. This phenomena is known as “labradorescence” and named after the mineral that is best known for producing it.
Amazonite is a trade name for a green to bluish-green variety of microcline feldspar that is often cut into cabochons, beads and tumbled stones. The green color is caused by trace amounts of lead within the mineral. A world-famous locality for amazonite crystals is in the Freemont, El Paso and Teller Counties area of Colorado where it is found growing with smoky quartz in cavities of igneous and metamorphic rocks. Although the name “amazonite” suggests that Brazil is a major source of the material, it is not found in the Amazon Basin.
Feldspars are the most abundant minerals, classified in two main groups, the alkali series and the plagioclase series. There are 100 million tones of feldspar reserves in 22 documented occurrences in B.C. All these deposits are commercial grade and so far gem quality materials (such as Moonstone, Labradorite, and Sunstone) have not been reported.