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Unakite


The pink and pistachio-green granitic gem material.


unakite

Unakite: Pieces of crushed unakite that could be used in a rock tumbler or used as a crushed stone at a construction site. The pieces shown here are about one to two inches across and were mined in South Africa.

What Is Unakite?

Unakite is the name used for a coarse-grained granitic rock that, after metamorphism, contains abundant pink orthoclase and pistachio-green epidote. These colors have helped it become a popular lapidary material. It is easily cut and polished to produce beads, cabochons, small sculptures, and other ornamental items. It is also a popular material for producing tumbled stones in a rock tumbler. Unakite is attractive, abundant, inexpensive, and frequently seen in the craft jewelry marketplace.

Unakite is sometimes used as an architectural and decorative stone. Slabs of unakite are used as flooring tiles, facing stone, stair treads, and windowsills. Its most prominent use is as a trimming to the front steps of the Smithsonian National Museum of Natural History in Washington, D.C. [1] It is also used as floor tiles on a landing at the south entrance.

Unakite has also been used as a construction aggregate. Crushed stone made from unakite has been used as road base material, drainage stone, unpaved road surfacing, and fill.




Geologic Occurrence

Unakite is a metamorphic rock that forms when granite is altered by hydrothermal activity. During metamorphism, plagioclase in the granite is replaced by epidote to produce a rock composed primarily of green epidote, pink orthoclase, and clear to bluish-gray quartz. [2] Unakite can also contain minor amounts of magnetite, chromite, ilmenite, apatite, zircon, and other minerals.

Unakite is found in the deformed rocks of convergent plate boundaries where deeply-formed granites have been metamorphosed and exposed by weathering and erosion. It occurs where nearby fractures have delivered the hydrothermal fluids that altered the granite.



tumbled unakite

Unakite as Tumbled Stones: A group of tumbled stones made from unakite mined in South Africa. They are about 20 to 25 millimeters in length along their maximum dimension. Photograph provided by RockTumbler.com.

Unakite Localities

Unakite is named after the Unaka mountain range of western North Carolina and eastern Tennessee, where it was first discovered and described. Similar material is found in many other locations. It is known as the Pompton Granite in a 1/2-square-mile outcrop in the Piedmont physiographic province of New Jersey. That small area has produced architectural stone that has been used in many prominent buildings of New Jersey and surrounding states. [3]

Many locations in the Blue Ridge physiographic Province of Virginia have been mined to produce unakite for construction, architectural, and lapidary use. Unakite has also been produced in South Africa, Sierra Leone, Brazil, and China.

Information Sources
[1] Granite GeoGallery, a page on the Geology, Gems and Minerals Subject Guide on the Smithsonian National Museum of Natural History website. Accessed in February, 2016.

[2] Mesoproterozoic Geology of the Blue Ridge Province in North-Central Virginia: Petrologic and Structural Perspectives on Grenvillian Orogenesis and Paleozoic Tectonic Processes by Richard P. Tollo, Christopher M. Bailey, Elizabeth A. Borduas, and John N. Aleinikoff; Geology of the National Capital Region - Field Trip Guidebook, United States Geological Survey Circular 1264, pages 17 to 75, 2004.

[3] The History of the Pompton Pink Granite: New Jersey's Prized Building Stone by Rich Volkert, New Jersey Geological Survey Information Circular, 2007.

Epidosite

Epidosite is a material similar to unakite, but with little or no pink feldspar. It is also an attractive material that is used to make beads, cabochons, tumbled stones, and other items. Its pistachio green color and crystalline texture cause many people to call it unakite, but epidosite is the proper name.

unakite cabochons

Unakite Cabochons: Two cabochons cut from unakite. The one on the left consists of approximately equal amounts of green epidote and pink orthoclase feldspar. It measures about 30 x 19 millimeters and is cut from a material with a very coarse grain size. The cabochon on the right consists mainly of epidote and has a much finer grain size. It measures about 39 x 30 millimeters in size.

Gemology of Unakite

Unakite is not seen in fine jewelry, seldom seen in commercial jewelry, but is a common stone used in craft jewelry. Unweathered, fine-grained unakite with mineral crystals less than a few millimeters in size is relatively easy to work. It is suitable for cutting by a person who has a small amount of lapidary experience. The hardnesses of the primary minerals (epidote = 6 to 7; and, orthoclase = 6) are close enough that severe undercutting or overcutting usually does not occur. It is only when the grain size is larger, or the piece being cut is very small, that differences in hardness will make cutting difficult.

Unakite cuts best using diamond abrasives but can also be cut or tumbled using silicon or aluminum oxide abrasives. It polishes well with inexpensive aluminum oxide polish, but tin oxide, cerium oxide, and titanium oxide also produce good results on a felt lap or in a rock tumbler.

In jewelry projects, unakite works best in pieces that will not be subjected to abrasion or impact. The hardness of orthoclase and epidote are low enough that they will show signs of wear when used in a bracelet or ring. These minerals also have perfect cleavage and could break with a moderate impact.

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