Spinel was once known as the great imposter of gemstones. Before the rise of modern gemology, spinel was often mistaken for rubies and sapphires. One of the most famous spinels of all time, a 170 carat red gem known as “the Black Prince’s Ruby,” has a place of honor on the Imperial State Crown of England. Once the crystal structure of spinel was understood, some mis-named ‘rubies’ were correctly identified as spinel, and as a result, the reputation of spinel suffered because they were simply not rubies.
In recent years, spinel has become valued in its own right as a rare and beautiful gemstone. The extensive treatment of some lower quality rubies and sapphires has helped focus more attention on the natural beauty of spinel, which is almost always untreated. Spinel occurs in a wide variety of colors, and is valued like many colored gemstones, using the same four Cs used to evaluate diamonds: color, clarity, cut, and carat weight.
How Spinel Color Affects Value
Like many colored gems, the most important factor in evaluating spinel is its color. Spinel is an allochromatic gem, meaning that when it is pure it is colorless. The color in spinel is dependent on the presence of trace elements like chromium, iron, and cobalt. Because of these slight impurities, spinel occurs in almost all colors, ranging from pink and lavender to red and red-orange, purple, blue, and even black, with pure green and pure yellow being notable exceptions.
Color in all gemstones is referred to in terms of hue (the basic color of the stone), tone, (the relative lightness or darkness of the hue), and saturation, (the intensity or purity of hue). Saturation is the color quality that will most greatly impact the price of spinel.
Because red spinel was often mistaken for ruby, red has traditionally been the most commercially valuable color. Red in spinel can range from purplish red to orangey red, with highly saturated reds being the most coveted. So called “ruby spinels,” with red hues, medium tone, and strong saturations, are among the most valuable of all spinels. Even so, a top quality five carat spinel is generally only worth about one tenth the price of an equivalent quality ruby.
The next most valuable color for spinel is blue, with colors that range from violet-blue to slightly greenish. Although most blue spinel generally occurs with lower saturations resulting in a grayish look and a more affordable stone, blue spinels that are colored by cobalt resemble blue sapphires, with highly saturated violet-blue to pure blue hues and medium tone. These cobalt blue spinels are especially valued for their rich, pure color.
Orangey red spinels with strong saturations and medium hues, sometimes called “flame spinels,” are the next most sought after color. Vivid orange and vibrant, hot pink spinel are equally coveted.
Purple spinel with strong saturation can be somewhat valuable, though darker violet, purple, and lavender stones tend to be in less demand, and command lower prices. Any spinel with low saturation, or a steely or grayish look is generally less sought after and accordingly lower in value.
Evaluating Spinel Clarity
Clarity in gemstones refers to the existence and visual appearance of internal inclusions, or small imperfections in the stone. Spinels are generally free of visible inclusions, but any inclusions that are visible to the naked eye can greatly decrease the value of the gemstone.
Occasionally, spinel can exhibit inclusions that reflect the gems distinct octahedral crystal growth, resulting in the appearance of fingerprint-like patterns. Another potentially valuable inclusion results in asterism, or the appearance of a star effect on the surface of the polished stone.
How Spinel Cut Affects Value
Spinel can be cut into a variety of shapes, though like many other colored gemstones, cushion shapes and ovals are popular. While some commercial quality spinel is cut into standard industry sizes, most higher quality rough spinel is cut into non-standard sizes to save the overall weight of the rough stone.
Standard-cut, or calibrated stones, are usually fabricated from medium to lesser quality rough material and is therefore more affordable. Calibrated spinel is generally fashioned into mixed-cut ovals measuring 6×4 mm and 7×5 mm, and is commonly used as center stones for rings.
Spinel Carat Weight
Like almost all gemstones, larger examples of spinel are simply more valuable than smaller ones. Because the larger sizes of spinel are rare, the per carat price rises considerably for stones greater than two carats. Fine examples of blue and pink spinel above five carats can be quite valuable, with the finest red spinel over ten carats fetching as much as $10,000 per carat.
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