On the Rocks: Color in Gems

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Almandine garnet
Almandine garnet crystals on gneissic matrix, Paraiba, Brazil. Eurico Zimbres and Tom Epaminondas, WikiMedia Commons

By Bob Jones

Of all the elements on the Periodic Table, a small cluster of transition metal elements play a major role in determining the value of many popular and valuable gems. For example, zoisite is usually an opaque pink thanks to manganese and green due to a trace of chromium. But zoisite variety tanzanite is an astoundingly wonderful and colorful gem.  The very valuable and vibrant blue to violet tanzanite is from Tanzania.

Few realize the great majority of the tanzanite found has a brownish tint because the trace vanadium in it has a valence of three electrons.

Impact of Trace Transition Metal Elements

The importance of trace impurities of transition metal elements in otherwise less attractive gems can not be overstated. Except for diamonds, which have high value even when colorless, it is the colorful gems that command higher prices in the market.

Another important aspect of tanzanite and many gems is the internal geometry of each crystal’s atomic structure. The distance between atoms in the structure determines the energy transition electrons must gain to move or shift. Tanzanite is a great example of this relationship between electrons, distances traveled within the molecule’s structure, and the amount of light energy is absorbed. Viewed in one direction a tanzanite crystal is blue. Turn it ninety degrees and its color is violet. Look down the length of the same crystal and you will see burgundy red. This is trichroism and is caused by the energy electrons require to move different distances in the mineral’s atomic structure when viewed from different directions.

Amber
Specimen of amber.
Hannes Grobe, WikiMedia Commons

As mentioned in an earlier On the Rocks column, the transition metal elements that cause more colors in gems than any other are iron, chromium, vanadium, and titanium. In some cases like ruby, the trace metal acts as a single chromophore locked in the geometry of the corundum’s atomic structure. In many other cases, it takes two transition metal elements working in concert with oxygen atoms to give a gem the color we adore. A good example of this is padparadscha orange sapphire, another corundum.

This type of sapphire is highly prized because of its rare color.

Most colorful opaque minerals get their color from a simple act of an included transition metal element, but the colors we see in many gems are due to much more complicated electron action.

Cause of Color in Common Gems

In other On the Rocks columns, we’ve touched on the more valuable gems like diamond, elbaite, sapphire, ruby, and amethyst. How about the cause of color in those more common colorful gems such as garnets, amber, sugilite, chrysoprase, coral and jade?

When the movie “Jurassic Park” came out, amber was suddenly the darling of the jewelry world. Everyone wanted a necklace, bracelet or ring of amber, especially with an embedded insect. We all know amber is a fossil resin produced by ancient trees buried and fossilized.

Color in amber ranges from pale yellow to a rich orange-yellow and even blue, which is natural and caused by the organic mineral’s chemistry. Its internal structure is such that its electrons, not impurities, simply charge transfer back and forth, causing the energy of light to be out of balance creating a visible color.

Chalcedony is normally gray-white but can exhibit all sorts of colors with the right impurities. With a little copper, it takes on a grand green color. With uranium oxide atoms included it will fluoresce bright green under short wave excitation. Infuse the same cryptocrystalline chalcedony with submicroscopic particles of a nickel clay and the lovely green gem gets a different name, chrysoprase.

Influence of Sugilite

Chrysoprase
A rough slab of chrysoprase.
Parent Géry, WikiMedia Commons

Of all the colorful chalcedony varieties, one that commands every jeweler’s attention and is thought to be sugilite is chalcedony infused with violet sugilite. The solid opaque form of sugilite gets its color from manganese and iron acting in coordination. But the gem form of sugilite-chalcedony is infused with microscopic particles of solid sugilite hence the violet color. This is far more valuable and popular than opaque sugilite as it is gem violet chalcedony.

When we think of jade we usually think of the massive green material so popular for thousands of years in China and now found throughout the world. Technically, there is no such mineral as green jade only nephrite and jadeite. In the orient, jade can be any green mineral.

Nephrite is the much more common jade mineral. Quantities are found in dozens of places throughout the world. It is formed through the combination of two minerals, tremolite and actinolite, developed in tight interlocking fibrous needles giving nephrite its toughness. The rich green color in nephrite is due to iron and sometimes chromium.

Jadeite is true jade, single mineral sodium, aluminum silicate sometimes with minor amounts of other minerals. Its color can range from porcelain white to yellow to green to violet with each color due to a different chromophore. Yellow-green in jadeite is due to iron and green jadeite is due to chromium. The rare violet jadeite is due to electrons transferring between iron, the valence of four and oxygen to iron valence three.

Mining Peridot Mesa

Here in Arizona, the green gem olivine variety peridot is mined by locals on the reservation from a volcanic outcrop called Peridot Mesa. Peridot can be one of two minerals, forsterite, and fayalite, so the gem’s color ranges from lovely green to light brownish green all due to iron. The volcanic olivine green sand from Hawaii is green due to a trace of chromium.

Sodalite is a very lovely massive blue mineral used in jewelry and carvings. It can be mistaken or substituted for lapis. Sodalite’s lovely blue color comes from oxygen substituting for aluminum or silicon atoms as shifting electrons use light energy.

There are two other blue minerals with similar names, lazulite and lapis lazuli. Lazulite is found in sharp dark bluish crystals and is not as rich a blue as the other mineral, lapis lazuli. Lazulite is found in different types of deposits, from pegmatites to metamorphic rock. Lapis gets it rich blue color from a charge transfer of electrons between iron to oxygen to iron. A pale blue type of lapis containing lots of calcite is often called denim lapis.

Lapis lazuli is the earliest known gem mined by humans and is found in the Sar-e-Sang (Sar-i-Sang) district of Badakhshan, northeast Afghanistan, the major source. It occurs in white limestone marble in pods and masses, sometimes as fine dark blue dodecahedral crystals. The color of lapis ranges from a distinct good blue to dark blue and has been cut, carved and worked for jewelry, royal regalia and art objects as long as humans have collected gems.

Odd Causation of Color

The cause of color in lapis is quite odd. Its chemistry is complex sodium, calcium, sulfur, aluminum silicate. The sulfur atoms in the structure are the cause of color as they absorb light energy and charge transfer their outer orbital electrons among themselves using longwave energy from the red end of the spectrum which makes blue light energy dominant. Because of the continued difficulty and rarity in mining lapis with an increasing demand for it, blue lapis is being simulated, dyed and otherwise treated, making it difficult for the average rockhound to identify natural specimens.

One of the largest gem families of species that is extremely popular, colorful, plentiful and useful are garnets. They range from very common and universally available like almandine to quite rare and valuable as gems like tsavorite, a variety of grossular.

There are nearly twenty recognized members of the garnet family, but only seven or eight are very popular and commonly used as gemstones, specimens and for industrial use. These are almandine, pyrope, spessartine, andradite, grossular, and uvarovite. Though there are colorless garnets, those with color have traces of transition metal elements in them including iron, manganese, chromium, vanadium or titanium.

This writer’s first productive collecting experience was in 1947 for red almandine garnets in

Sugilite
Sugilite crystals present within a matrix of undetermined and unstable brown clevage, with lacquer applied to specimen, collected at Wood’s Mine, New South Wales, Australia.
Matthew Goodwin, WikiMedia Commons

Connecticut’s well-known source, now Green’s Farm, Roxbury, which yielded huge quantities of dark red almandine garnet in a schist deposit. It produced garnet for grinding and sandpaper as well as collector specimens. When the state of Connecticut selected a state gemstone it was almandine garnet from this deposit.

Members of the garnet family are the more common state gemstones. Among the states that have garnets as their state gem are, New York choosing the red almandine from the Gore Mountain area, Idaho choosing star garnets with rutile inclusions causing the star pattern and Vermont selecting grossular, certainly because of the lovely intense orange garnets from the Eden Mills area.

Elusive Tsavorite

While fine garnet crystals are available to virtually all collectors, the one gem garnet that is out of reach for most of us is green tsavorite crystals found by my friend Campbell Bridges in Africa.

The garnet equals and often rivals the finest emeralds for color, clarity, and value. Like emeralds, tsavorite crystals gain their color from trace inclusions of chromium and vanadium.

Campbell found his first deposit in Tanzania in 1967. Not allowed to mine by Tanzanian authorities he found a second gem deposit in Kenya near Tsavo National Park hence the name. The tsavorites were successfully marketed by Tiffany’s. My wife, Carol, and I were most fortunate to get to know Campbell when he was in Tucson. Unfortunately, Campbell was murdered in Africa by thugs, for control of his gem deposits.

There are far more inexpensive gem crystals available to rockhounds than are listed here. Their sharp crystal form, availability to the collector, variety of color and occurrences makes garnets an ideal family of minerals to collect.

My only hope is you will enjoy collecting the garnet varieties and all the other less valuable but beautiful whose colors are due to transition metal elements or other elements on the periodic table a very useful tool for all mineral collectors.