For centuries, Spinels have been used to replace high quality gemstones; in particular, spinels were substituted for rubies and sapphires that were seen as too valuable to put at risk. And even now, Ruby Red and Deep Blue Spinels tend to hold the highest value.
Zircons have suffered much the same fate, but not for the same reason! The similarity in the name "Zircon" and that of the man-made diamond substitute, Zirconia (actually Cubic Zirconia, or CZ), made Zircons undesirable. And that was a very sad waste!
Both Spinels and Zircons are finally being recognized for the gorgeous, unique gemstones that they are!
Spinels can range in color from the softest pink to blood red, from pale blues and purples to midnight shades and black, and even golden tones can be found! They are a SINGLE refractive gemstone - a minority in the gemstone world - and have an RI range (single number), 1.71 - 1.80, with very specific stops along the way. These would be the identifying RI of the exact variety of Spinel; 1.80 - is the Gahnite, a blue-blue/green color; 1.76 would be a blue spinel (no green tone) known as Gahnospinel; and 1.718 should be the reading for all other spinels; a synthetic spinel will register as 1.73. Interestingly, the two gemstones that Spinels have stood in for over the years - Rubies and Sapphires - are Corundums, and their refractive index, although a double, is very similar - 1.76-1.77. And, while Corundums rank a 9 on the Moh's scale, Spinels are relatively sturdy gems, as well, ranking an 8, the same as Topaz!
Zircons have been treated in a similar manner, and have often been confused with the diamond simulants. In reality, Zircons are a species in and of themselves, and actually have varying groups within. Unlike natural diamonds, Zircons are DOUBLE refractive, and can be broken into 3 groups using their RI - low (1.810-1.815), medium (1.875-1.905) and high (1.925-1.984). Unlike the record-setting 10 on the Moh's scale that a Diamond possesses, Zircons rank only about 7.5 in hardness.
But Zircons, like Spinels, are found in a wide range of colors, naturally - from red to yellow to brown, and blue to clear or white! Like many gemstones, the color of the Zircon is often enhanced by heat-treating the stone. Heating a stone provides permanent, consistent color change throughout the stone (as opposed to topical treatments - aka - vapor deposition; see additional guide related to gemstone treatments.)
While the world has been busy using Spinels and Zircons as substitutes for other gemstones, they have missed out on the beauty these stones have to offer! They are both finally being appreciated for their own strengths and attributes, and the prices of these stones reflect that new appreciation!