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Corundum (ruby, sapphire)

Corundum (ruby, sapphire)


Hardness: 9

Density: 3.97 – 4.05

Chemical symbol: Al2O3

Crystalline system: trigonal

Color: white, red, blue, pink, orange, yellow, brown, colorless. Transparent, glassy glitter.



Quick identification:


Glass is much softer. Spinel is softer but with basic equipment it is hard to tell the difference. Unpolished spinel is distinguished based on different crystalline systems. Garnet is much softer and has a different color tone to ruby. Cubic zirconia has a higher birefringence than a colorless sapphire.


Detailed identification:


Laymen without any experience or special equipment are not able to distinguish corundum from other gem stones, especially imitations. The main method of identification used is color differentiation (e.g. red stones are assumed to be rubies, garnet, red glass and so on) and then by using a process of elimination we can narrow it down. (Glass is eliminated by hardness tests, garnet is eliminated based on the specific garnet color and so on). Sometimes the differentiation process is extremely difficult – for example, after centuries, the English royal crown was found to be decorated with spinels and not rubies.


How to determine the authenticity of corundum (ruby or sapphire) without equipment:

(quick identification – may not be reliable)


a) Test the hardness of the stone - it should make a scratch on glass.


b) To distinguish cubic zirconia from corundum, check the birefringence of the gemstone - cubic zirconia has a much higher birefringence.


c) Natural stones always have defects, inclusions, refractions and so on, which are visible under the microscope or sometimes even with the eye. Artificial stones are flawless. However, if the gem contains gas bubbles, it is glass.



Interesting facts about corundum (ruby, sapphire):


1. The word ruby comes from the Latin word "rubens," meaning "red."


2. All varieties of corundum except red ones are sapphires.


3. Ruby is known as the jewel of love. In the past, women bought jewelry with rubies as they believed it created an aura of beauty around them.


4. The most valuable rubies are considered those with a dark red color, the so-called "pigeon blood" ones. Rubies can come in different colors, from light pink to very dark red.


5. The king of Burma (now Myanmar), from where top-quality rubies come, was known as "Sir Rubin." This is because all rubies of 6 carats and over were declared his property.


6. In the Bible, the ruby is the gemstone that is mentioned the most.


7. Rubies were often a present for high Chinese imperial officials, because it was believed that they gave the gift of leadership and guidance.


8. In Burma, it was believed that if soldier put a ruby under his skin, he became invisible. Jews believed that a dragon engraved on a ruby brought prosperity and health.


9. The largest ruby in the world is in the American Museum of Natural History and weights 23 carats.


10. In ancient times, rubies were sometimes laid under the foundations of buildings as it was believed that rubies tightened their structure.


11. The only difference between a ruby and a sapphire is the presence of chromium oxide in rubies.


12. The word sapphire comes from the Greek "sappheiros," which means "gem."


13. The most common color of sapphires is blue.


14. Sapphires are a gift for the 5th, 23rd and 45th wedding anniversaries.


15. For hundreds of years, sapphires have been associated with wisdom, fidelity and purity. Sapphires were often worn by religious dignitaries to protect them from physical attacks and envy.


16. The ancient Persians believed that the earth rested on a giant sapphire whose reflection made the sky blue.


17. The largest cut and polished sapphire is the gigantic "Millennium Sapphire" at 61.500 carats.


18. According to legend, Moses received the Ten Commandments on sapphire tables.


19. In the Middle Ages, it was believed that the sapphire had healing effects and that they could remove all dirt and foreign bodies from the eye.

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