|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.|
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.
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.
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.
ONE HALF OF A SPINEL OCTAHEDRON FROM CALIFORNIA.
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.
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.