(MgAl2O4) is a much maligned gem mineral that is often thought of as
a corundum simulant because in red and blue varieties it can closely resemble ruby or
sapphire. It is, however, an outstanding gem material in its own right; it is hard (8),
possesses an intermediate to high R.I., shows good dispersion, and is available in a wide
variety of colors. Many of the worlds most famous large
"rubies" are, in fact, red spinel (e.g. Black Princes' Ruby and Timur Ruby, both in
British Crown Jewels). Such stones were once referred to as Balas Ruby, a
term that is not used today but is synonymous with red spinel. The confusion is
well-founded; the finest red spinels come from the same gem gravel deposits in Myanmar
that are renowned for "Burma" ruby, and both share somewhat similar optical and
Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.
"Spinel" is synthesized in the lab by the flame fusion process (Verneuil method) and is widely used as a corundum, diamond, and chrysoberyl imitation. Virtually any color of the rainbow can be made, including those not found in nature (yellow), but all differ from natural spinel in chemical makeup (more Al than Mg in comparison to naturals). This difference is reflected in the R.I. and S.G., which are consequently valuable properties in distinguishing naturals from synthetics, or synthetic spinel from corundum (see below).
Gem spinel forms in the same rocks and by the same processes that produce gem corundum. The most common occurrence of gem spinel is in metamorphosed impure limestones (marble), but it has also been found in pegmatites (Zn-rich varieties, these are rare). The principle historical producer of spinel was Burma (Myanmar). Presently most production is from the gem gravels of Sri Lanka and Thailand.
Little info.. As with other colored stones, depth and purity of color and clarity are highly prized. Reds and pinks are apparently the most expensive, closely followed by lavender and blues. The price trend with color seems to follow sapphire. Good gem spinel in any color (particularly pinks and reds) in sizes of 3 carats or above is very rare (rarer than ruby or sapphire). Several authors state that given the rarity and beauty of good pink and red spinel these stones are highly undervalued and should continue to increase in price. Prices have historically suffered from confusion and comparison with corundum and synthetic spinel. Detractors from value are brownish or Grey overtones, or a muddy color, particularly in reds and blues.
Prices for 3-5 carat or stones of top quality where in the following ranges in 1990: