Natural Spinels: Originally, Spinels and Rubies were considered as the same stone because Natural Spinels were found in the same locations as Corundum. The red color species of Spinel were misidentified as Ruby and other colors of Spinels were as Sapphires. Even for centuries, the magnificent red gemstone of 5.08 centimeter popularly known as “Black Prince Ruby” at the center of the British Imperial Crown was thought to be Ruby only but studies have recently shown that the stone is Spinel actually.

British Imperial Crown, Picture Courtesy- Wiki Pictures

The most fascinating features of Spinels are:

• They are singly refractive. Gemstones other than Diamonds, Spinels, and Garnets are double-refractive.
• There is a complete absence of cleavage.
• They formed as a cubic crystal like a Diamond.
• Spinel is seen in a wide range of colors: red, lavender, violet, blue, green, brown, black, and grey.
• Spinels have long Toughness & Durability.

 

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Spinel- Gemological Properties

Chemical composition	Mg(Al2O4) Magnesium aluminum oxide
Crystal system	Cubic
Habit	Octahedral, contact twins
Cleavage	Imperfect
Fracture	Conchoidal, uneven
Hardness	8
Optic nature	Isotropic
Refractive index	1.712 – 1.736
Birefringence	None
Dispersion	0.026
Specific gravity	3.58 – 3.61
Lustre	Vitreous
Pleochroism	None

The most significant deposits of Spinels occurred in Myanmar (Burma), Sri-Lanka (Ceylon), Cambodia, Thailand, Brazil, Madagascar, Nepal, Tanzania, Tadzhikistan, Afghanistan, and Australia. Spinels come in a variety of colors which range from blue spinel, red spinel, green spinel, yellow spinel, brown spinel, black spinel, orange spinel, pink spinel to violet spinel. But the rarest Spinel is Cobalt Blue Spinel which has a rich deep blue color.

Here, You can also check out :SPINELS: HISTORIC UNDERDOGS, MODERN DAY MARVELS.

Central Asia Corundum and Spinel Reserves, Picture Courtesy- Ruby Sapphire.com

Types of Spinels: A variety of names has been given to the different members of the Spinel family. Some are:

1. Balas Spinel: These are a pale-red variety of Spinel is called Balas Spinel. In the past, it was mistakenly known as Balas Rubies.
2. Flame Spinel: These are bright orange and orange-red varieties of Spinel. In the past, it was known as Rubicelle.
3. Hercynite Spinel: The color of Hercynite ranges from dark-green to black, due to the presence of iron content.
4. Pleonaste Spinel: The color of Pleonaste is also dark-green to blackish opaque, but is a little lighter than Hercynite. It is popularly known as Ceylonite.
5. Gahnite Spinel: These are blue, violet, and dark-green to black varieties of Spinel. Sometimes also known as Zinc Spinel.
6. Gano Spinel: The color of this variety is similar to Gahnite but less dense colors are found.
7. Picotte Spinel: These are yellowish-brown, dark-green to black variety of Spinel. Sometimes known as Chrome Spinel.

Synthetic Spinels: Spinels can be synthesized by using three methods:-

1. Verneuil (flame-fusion) method
2. The flux-melt method
3. Czochralski (pulling) method.

 

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The Flame-Fusion Method: This method of producing Synthetic Spinels occurs accidentally while producing synthetic Corundum in 1908. A Natural Spinel has a ratio of MgO to Al2O3 is 1:1. Verneuil Synthetics are produced by changing the ratio to 2:1 (doubling the amount of alumina). The increased amount of alumina resulted in an increased Refractive Index (1.727) and Specific Gravity (3.64).

Properties of Flame Fusion Synthetic Spinel
Color	Coloring agent(s)	RI	SG	Other diagnostic tests
Colorless	none	1.728–1.740	3.65–3.80	LW UV: green; SW UV: white/blue
Red	Cr3+	1.722–1.725*	3.58–3.60	curved striae; spectrum; fluorescence
Pink	Cu	1.727–1.740	3.65–3.80
Yellow	Mn	does.	do.	LW/SW UV: green
Emerald green	Mn + Co3+	do.	do.
Tourmaline green	Cr3+	do.	do.	spectrum
Beryl green	Cr3+ + Mn	do.	do.	spectrum
Zircon blue	Co3+ + Cr3+ + Ti	do.	do.	spectrum
Sapphire blue	Co3+	do.	do.	spectrum; fluorescence; CCF
Amethyst violet	Co3+ + Mn	do.	do.
Alexandrite color change	Cr3+ + V	do.	do.	spectrum
Lapis lazuli imitation	Co3+	1.725	3.52	spectrum; C

Diagram showing how Synthetic Spinels are produced under Verneuil Method.

The Flux-Melt Method: It was observed that by the Flame-Fusion method red spinels of larger quantity cannot be produced. To produce, bigger red even the blue Spinels the Flux-Melt method is used. The synthetics produced by this method are true synthetics as they are produced by a 1:1 ratio of alumina and magnesium.

Properties of Flux-Melt Synthetic Spinel
Color	Coloring agent(s)	RI	SG	Other diagnostic tests
Red	Cr3+	1.716-1.719	3.58-3.62	LW/SWUV: distinct red-orange/red residual flux inclusions
Blue	Co2+ + Fe2+	1.719	3.58	Spectrum; SWUV: inert; LWUV: Weak red residual flux inclusions

The Czochralski (pulling) Method: This method is a recent development (2007). The production of red to pink Synthetic Spinels is done by Czochralski (pulling) method.

Difference between Natural and Synthetic Spinels:-

                                                                   Picture Courtesy – GIA

1. Microscopic Evidence:  Synthetic Spinels have flux or some growth viewed under careful microscopic observations while Natural Spinels have a lot of natural inclusions or at least some when viewed under a microscope. The following pictures would explain this concept more clearly. 

2. Magnetic Properties: Natural and Synthetic Spinels can be separated based on their different magnetic properties. A Natural Spinel has iron content hence; they show weak to moderate responses to magnets. While Synthetic Spinels have no iron content in them as a result, they do not any respond to magnets. Some light-colored and orange color of Natural Spinels is colored due to chromium, which causes it to be repelled by a magnet.

3. Refractive Index and Specific Gravity:  The Natural Spinel has a Refractive Index of 1.72, in the case of Red Spinel it is 1.74 and in Zinc-rich Spinel it can be1.805 (rare to be found). Synthetic Spinels has a noticeably different R.I. of 1.727-+0.001. The specific gravity of Natural Spinels has S.G. ranges 3.58-3.61, Zinc rich Spinels have S.G. as high as 4.40. Synthetic Spinels has S.G. ranges between 3.63- 3.64.

4. Fluorescence under UV rays: A Natural Spinel has the strongest fluorescence under a long wavelength and weak to absent under a short wavelength. While, Synthetic Spinels have strong fluorescence under a short wavelength and may often a bright blue, chalky green, or red fluorescence.

Synthetic Spinel showing a neon red color under short UV rays. Picture Courtesy- Ruby Sapphire.com

5. Price: There is a big difference between the price of Natural and Synthetic Spinels. A Natural Red Spinels range starts from 100$/carat and goes till 5, ooo$/carat. Price varies from color to color; red is the costliest, followed by blue, then pink, and in the last multi-colors.  Synthetic Spinels are very cheap. They are easily available in the market at a price of 1$/carat-2$per carat.

6. Synthetic dark Red Spinel is apparently not manufactured in the quantities that most of the colors. Most dark synthetic red stones are corundum.

Conclusion:

S.No.	Basis	Natural Spinels	Synthetic Spinels
1.	Price	100$/carat-5000$/carat	1$/carat-2$/carat
2.	Natural Inclusions	Yes	No,  growth signs can be seen
3.	Strongest         Fluorescence	Under long wavelength	Under short wavelength
4.	Refractive Index	1.72
5.	Specific Gravity	3.58-3.61	3.63-3.64
6.	Response to Magnet	Yes	No
7.	The ratio of Al: Mg	1:1	2:1 under flame fusion method

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