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The Basics for Purchasing Spinel, Sapphire and Ruby

Blog / 06.01.2016

Thousands of years ago, gems were classified solely by their color. Gemstone traders of this era would have considered spinel and ruby to be the same stone.

 However, with the advancement of science and understanding of the chemical and physical make up of gemstones, we now know there is a vast difference in characteristics between ruby and spinel. These variations or “clues” help the everyday gem buyer and dealer to identify stones and determine their quality.

Below are some easy clues you can use the next time you purchase sapphires, rubies or spinel – the most popular three stones to purchase in Bangkok.

These simple tips will allow you to begin correctly identifying these stones from their synthetic counterparts and ultimately, walk away with confidence from your transaction.

 Getting to Know Inclusions

A good starting point to learn about the value of a stone is to be able to identify its most common inclusions. For gem quality stones such as spinel, sapphire and ruby – most stones you’ll see will have eye-clean inclusions, meaning they will only be visible under magnification.

Included crystals and fingerprint inclusions are the most common inclusions in rubies. Though they occur in most gems, their influence on value is dependent upon their size, number, position and color. They also provide clues that help identify synthetic from natural gemstones. 

  • Intact rutile silk can prove that corundum has not been heat-treated.
  • Dark crystals located directly under the table of a ruby will reduce its value.
  • Corundum typically displays distinct patterns of color zoning.
  • Fluid or crystal inclusions, such as fingerprints, can occur in groups. Called fingerprints for their resemblance to human fingerprints. 

Synthetics & Imitations


Synthetic corundum can be made by the greatest variety of processes. The Czochralski method or pulling, flame fusion, floating zone, hydrothermal and flux growth are some examples. Due to this, synthetic corundum is available at a wide variety of price levels.

Triplets or doublets can made be of the same material or a combination of natural and synthetic stones to mimic rubies and sapphires. These are easy to identify as they have an obvious layering when viewed with a loupe.

 In recent years, reports about “glass-filled rubies” being sold as natural without disclosure have popped up in the US and elsewhere. The continued production of these goods undermines confidence in rubies. 

  • Ruby remains the most difficult to distinguish from its synthetic counterparts.
  • Balas ruby – a misnomer for red spinel – has long been used to imitate ruby. 


Nowadays, Thailand has a low output of ruby rough, however it remains at the center of heat-treatment. For rubies, heat is used to remove purple and brown overtones. The process is also used to lighten or eliminate the blue color in dark blue sapphires, remove blue hues from purplish rubies and intensify red colors.

Cavity filling (usually with plastic or glass) is common in rubies, and sometimes in sapphires. They can be distinguished by differences in luster and hardness. This process fills and seals voids, improves appearance and adds weight.

Chromium and titanium lattice diffused stones form a shallow layer of red and blue color, which can be identified when viewing the stones in liquid and diffused light. Beryllium treated stones are harder to detect as they have good depth of color. 

  • Dyeing is common in very low quality corundum, and its color can be very uniform and deep in stones with surface reaching fractures.
  • Irradiation is common for fancy colors.
  • It’s safe to assume that almost all sapphire and ruby have been treated unless otherwise proven.
  • Scratches, chipping, repolishing or cutting can remove color from latticediffused rubies and sapphires.


Color & Value


To be considered a fine color, gem colors of spinel, sapphire and ruby must be of high saturation and in the medium to dark tone range valued in the trade.


  •  Red fluorescence in rubies enhances the stones vibrant red color - increasing its value.
  • Best sapphire colors are velvety, violet blue to blue, in medium to medium dark tones, with vivid saturation.
  • Best ruby colors range from medium to medium-dark tones of pure red to slightly purplish red, with vivid saturation.
  • The best spinel colors can rival that of ruby; orangey to purplish red and pure reds of medium-dark tone are the most valuable.
  • High quality gems, like rubies and sapphires are not often cut to standard industry dimensions, as too much weight would be lost.


Lastly, lighting is also a factor that affects appearance of different colors. Using them to your advantage can help judge color quality.

Blue, green, violet gems look best under fluorescent lighting found in most offices. Red, orange, yellow gems look best in incandescent light, such as candlelight.

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