Gem Profile- Ruby Zoisite

While deciding how to begin this article, the Shakespearean quote, "A rose by any other name would smell as sweet," came to mind. What matters is what something is, not what it is called.

Let me explain some terminology briefly. As a rockhound who studies mineralogy and geology, I sometimes struggle to label something I would call a "rock" as a gemstone.

A "rock" is a combination of two or more minerals, while a "gemstone" is the term the jewelry industry uses for a specific mineral.

There are also many "rocks" accepted as gems due to their unique appearance. Some people call these "semi-precious," but I prefer the term "gem-rocks."

("Semi-precious" refers to all stones except the traditional "precious" stones: diamond, ruby, sapphire, and emerald).

A Closeup of Cathy Whitten's Ruby Zoisite Necklace

Ruby zoisite is a vibrant, metamorphic gem-rock composed of at least three minerals: corundum, epidote, and tschermakite (more commonly known as hornblende).

The apple-green shades come from chromiferous zoisite. Black and dark green spots and striations are edenitic hornblende. The reddish-pink areas are corundum, or ruby.

The geological name for African ruby-in-zoisite is Anyolite, derived from the Maasai word for green.

Dark green Epidote Crystal Fan specimen from Peru. Dale Armstrong, private collection.

Ruby Zoisite was first discovered in Austria in 1805 and named after Baron Sigmund Zois, the naturalist who identified it as a new mineral.

The better-known East African deposits were first documented in 1904 by a German military commander and scholar, who collected material named after himself, "Merkerstein." His work was never published, and the location was lost.

Around 1949, these "ruby-in-zoisite" deposits were rediscovered in Tanzania. Early miners mistakenly valued the ruby, but with all three components being dense metamorphic rocks, separating the ruby from the zoisite profitably was impossible.

Although very few gem-quality rubies were found intact, African Ruby Zoisite gained popularity in jewelry making, also known as Tanganyika Artstone.

Most ruby zoisite on the market today comes from the Longido mine at the foot of Mt. Kilimanjaro in Tanzania. It is plentiful, has a combined Mohs hardness of 5 to 7, and polishes well.

Flowers carved from Ruby Zoisite, Longido Mine, Tanzania, Africa. Notice the crystal structure on the bottom left flower (upside-down). - Dale Armstrong, private collection.

The mineral Zoisite occurs in a wide range of colors, including green, brown, pink, yellow, gray, colorless, and shades of blue to violet.

When natural reddish-brown zoisite crystals are artificially heated to bring out blue and violet hues, the resulting gem is called Tanzanite, rather than blue-zoisite. (We will discuss Tanzanite in more detail later.)

Zoisite formations are found in many places worldwide, including Africa, Austria, India, Pakistan, Switzerland, and the United States.

Ruby Zoisite Pendant by Teresa McMahon.

Ruby zoisite is not rare, so imitation is unnecessary.

However, there are two gem-rocks that look similar and are often mislabeled. One is from Mitchell County, North Carolina, and is an emerald green form of Actinolite called Smaragdite, which can contain red, pink, and blue corundum. Smaragdite can be difficult to polish due to varying mineral hardness.

The other material often mistaken for ruby zoisite is Ruby Fuchsite, which will be the focus of next week's Gemstone Profile. Comparison photos of all three materials will be provided.

Rubies and sapphires encased in Smaragdite from Mitchell Co, NC. - Dale Armstrong, private collection.