SPINEL: THE FORGOTTEN GEMSTONE



THE BLACK PRINCE'S RUBY DISPLAYED ON THE FRONT OF THE IMPERIAL STATE CROWN OF GREAT BRITAIN.  PHOTO PUBLIC DOMAIN FROM THE CROWN JEWELS OF ENGLAND.

I attended a recent show and one of the vendors had a very nice selection of faceted, gemmy spinels.  In fact, several displayed spectacular cuts.  I was further impressed by the specimens when I learned they were natural spinels rather than laboratory-produced crystals.  But, I really had to rattle the back part of my mind to even recall something about the mineral!  I actually had not thought much about spinel for years.

I don’t remember much about spinel from my undergraduate mineralogy class except seeing some very hard octahedral crystals rolling about in a white specimen box.  At first glance it was easy for me to get the crystals mixed up with magnetite octahedrons and I sort of wondered if there was a relationship.  But then, something in my brain said “wake up Mike, use a magnet” to weed out the spinel.  I never used the magnet since I don’t recalling needing to identify spinel on any test!  In fact, I don’t believe that I ever observed or identified the mineral in the field.

Later in life essentially the only thing I knew about spinel was that for years the royal British jewelers were a bit confused about gems in the crown jewels!  The Black Prince’s Ruby and the Timur Ruby (~350 carats), part of the crown jewels, are actually red spinels; however, before modern gemological tests were readily available most red–colored gems were termed rubies.  The Black Prince’s Ruby is ~170 carats in weight but is more of a cab or blob and is not faceted.  It prominently sits on the front of the Imperial State Crown, which in turn may be seen by visitors in the Tower of London Jewel House.  The Samarian Spinel is part of the Iranian Crown Jewels and may be the largest spinel gem in the world (~500 carets).  Legend has it this particular spinel adorned the biblical Golden Calf, constructed by the Israelites (for worship) while spiritual leader Moses was receiving the Ten Commandants.

A recent advertisement came across my computer screen extolling the thought that black spinels were a wonderful substitute for more expensive “black diamonds”.  That blurb intrigued me even more so I begin to dig through the literature, and also remembered that somewhere in the recesses of my collection was a specimen of spinel.  

 

Spinel, a magnesium aluminum oxide (MgAl₂O₄), comes in a variety of natural colors with nice stones being colorless (rare), red to pink (probably due to trace chromium), orange to purple (probably due to trace iron and chromium), black (probably due to trace iron), and blue (probably due to trace iron and cobalt).  A jeweler told me that very few faceted spinel gemstones are enhanced; occasionally, some are heated to improve the color. As noted above, red spinel is often confused with ruby, a red aluminum oxide (Al₂O₃) with trace chromium supplying the color in both.  In fact, they often occur together with ruby forming after the available magnesium is “used up”.

GEM RED SPINEL FROM THE GEM AND MINERAL COLLECTION OF THE SMITHSONIAN'S NATIONAL MUSEUM OF NATURAL HISTORY. PHOTO COURTESY OF DANE A. PENLAND.

 

Spinel is a hard mineral coming in at 7.5-8 on the Mohs’ scale and forms in the isometric system; most crystals are octahedrons.   It is translucent to transparent to even opaque and has a vitreous luster.  It does not cleave. As I understand it, most gem spinels are found in alluvial gravels with the finest red, pink and orange specimens coming from Myanmar (Burma).  Sri Lanka (Ceylon) is a major producer of blue and violet stones; however, it appears that new deposits have been located in Tanzania.

One of the unfortunate facts (or depending upon your point of view, fortunate) is that spinel is easily synthesized and colored and many people simply do not appreciate the beauty of natural spinel; they may not even realize that “natural” spinel exists!  In fact, some of the colors in synthetic stones do not appear in nature and virtually all clear crystals marketed as spinel are synthetic.  Synthetic spinel also is often used to replicate other gemstones such as diamond, ruby and sapphire---know your dealer! Have an old high school class ring with a big colored stone?  There is a good chance the colored stone is synthetic spinel.

Spinel is a member of the “Spinel Group” of minerals crystallizing in the isometric system and with a general formula of AE₂O₄.  A represents various metallic cations.  Minerals in this group include the Spinel or Aluminum Series where E is Aluminum, the Magnetite or Iron Series where E is iron (Fe), and the Chromite or Chromium  Series where E is chromium._.  The Aluminum Series includes the gem spinels, but at times the magnesium of spinel (the A) may be replaced by zinc (the mineral gahnite), iron (the mineral hercynite), or manganese (the mineral galaxite).  The best known mineral of the Iron Series is magnetite (Fe₃O₄) while chromite (FeCr₂O₄) is a representative of the Chromium Series.

MAGNETITE OCTAHEDRONS FROM MAGNET COVE, ARKANSAS. WIDTH OF SIGLE CRYSTAL ON RIGHT ~1 CM.

Spinel is found in both igneous rocks (basalts, peridotites, kimberlites) and high temperature contact metamorphic rocks (especially hornfels and marbles).  Some of the most beautiful specimens that I have seen in museums are where a crystal of red spinel is perched on a metamorphic white marble.

The specimen in my collection (black and opaque probably due to iron) was purchased at a show and labeled “Huntington Lake, Fresno County, Calif.”  As best that I can tell this spinel occurs in “a deposit of pre-Cretaceous crystalline limestone changed to marble and containing several smaller bodies of calc-silicate hornfels… It is in the high Sierra Nevada, 3 miles north of the east end of Huntington Lake, at an elevation of 8500 to 8800 feet. As mapped, this limestone pendant is over 10,000 feet long and from 1250 and 3000 feet wide… the contact-metamorphic rocks formed where the Sierra Nevada batholith intruded a region deeply covered by older sediments, of which the original upper portions have been removed by erosion, including glaciation” (Logan, 1947).  Pink, green and lavender crystals also have been collected from this locality.

SMALL MASS OF BLACK SPINEL OCTAHEDRONS COLLECTED FRESNO COUNTY CALIFORNAI.  WIDTH OF SPECIMEN ~5.2 CM.  LARGE CRYSTAL UPPER RIGHT IS BELOW.

 

ONE HALF OF A SPINEL OCTAHEDRON FROM CALIFORNIA.

I certainly have not collected crystalline spinel (Spinel Series) anywhere in Colorado and Eckel (1997) stated that “spinel…has seldom been observed in Colorado”.  However, he also noted that Spinel Series minerals “are common accessory constituents of mafic igneous and metamorphic rocks… and are much more widespread than indicated by the literature.  Much of the spinel…occurs associated with Precambrian massive sulfide deposits”.
Gahnite, the zinc-dominant member of the Spinel Series, “is commonly found in Colorado in Precambrian metamorphosed, base-metal sulfide deposits and in a few pegmatites of granitic composition…It is moderately common in such deposits in the Front Park, and Sawatch Ranges”.  One of the local rock and mineral shops has several specimens of gahnite on display and for sale; however, the mineral is non-crystalline and is a granular (small) blackish (maybe dark, dark green), smear on country rock.  I certainly would not be able to identify it as gahnite without its label, or perhaps by using a microprobe stuck in my back pocket.

Several gem dealers refer to spinel as the “forgotten gemstone”.  It certainly was an import gem at one time---witness the crown jewels.  However, with the advent of synthetic spinel the stone seemed to decrease in value.  My suggestion is that readers ask their dealers for a peek at the real thing---you will be surprised at the gem’s beauty.

Rich and rare were the gems she wore,

And a bright gold ring on her hand she bore.

Thomas Moore

 

REFERENCES CITED

 

Eckel, E. B. (and others), 1997, Minerals of Colorado: Fulcrum Publishing, Golden, Colorado.

 

Logan, C. A., 1947, Limestone in California: California Journal of Mines and Geology, v. 43, no.3.