For those born in July, your birthstone is the resplendent ruby. Here, Gem-A tutor Lily Faber FGA DGA EG delves into the properties and folklore of this brilliant red gemstone.
The ruby slippers, a ruby red apple, ruby red lips. For those interested in gems and jewellery, and even those who aren’t, rubies are synonymous with a rich, red colour.
"The Sunrise Ruby"
Burmese ruby weighing 25.59 carats Sold for $30,335,698 ($1,185,451 per carat), at Sotheby’s Geneva, May 2015. ©Sotheby’s
What many don’t know is that the ruby is a member of the corundum gem species along with sapphires, the difference being colour and colouring elements. Rubies are specifically red, coloured by chromium, while sapphires have more flexibility with their colours (including blue and any other colour aside from red). The most valued red is called ‘pigeon’s blood’—a deep, rich red that is seen to be the purest iteration of the colour.
It is believed in some cultures that rubies bring prosperity and protection to those who wear them. Rubies are also the stone of passion, associated with love. It has even been said that rubies should be rubbed on one’s skin to promote or restore your youth.
Historically, all red gemstones were called a ruby until the late 18th century when gemmology developed as a science and people were able to distinguish between various species of red stones. In fact, some of the most famous rubies, including the Black Prince’s Ruby and the Timur Ruby in the Crown Jewels, are in fact red spinel.
Crystals and Inclusions
The most common crystal habit for a ruby is a flat, tabular hexagonal shape that can be either sharp or rounded on the edges. There may be raised, triangular growth marks on the top or bottom pinacoids of the crystal, as well as some lamellar twinning lines on the sides of the crystal. These are distinctive crystals and can be readily recognised. Twinning occurs when a crystal changes its direction of growth during formation either once or multiple times.
Ruby, crystal inclusion with twinning and twinning host. Photo by Pat Daly.
Rubies can contain a large variety of inclusions. These can be crystals, feathers (partially-healed internal fractures), silk (long, thread-like rutile inclusions), evidence of the lamellar twinning in the form of twin planes, and hexagonal colour-zoning. Silk, which forms in three directions at 120 degrees, can cause an optical effect known as asterism (a bright star that appears in the stone), which is highly valued in natural rubies. You can often tell if a ‘star-ruby’ is a natural one as opposed to a synthetic one because the star is not always centred, the arms of the star are slightly crooked or diffuse and the cabochon has a rounded or deep base to preserve yield, therefore raising the price of the stone.
Pre-treated and treated ruby. Photo by Pat Daly.
The most common treatment to corundum as a whole is heat treatment to enhance or remove colour. For rubies, brown and blue tones are often removed by this method. Additionally, it is important to watch out for rubies that have been fracture-filled with lead glass to improve colour and clarity. This treatment is becoming more and more prevalent, so caution is advised when buying rubies. The lead glass leaves tell-tale signs, however, so look for surface-reaching fractures, a difference in surface lustre between the glass and the ruby, and a blue flash within the ruby as well as bubbles, both of which are confined to the fractures inside of which the glass sits. It is not advisable to heat these rubies as the lead glass has a low melting point and will leak out of the stone.
Heat-Treated Crystal and Feather Inclusion. Photo by Pat Daly.
Care and Caution
Rubies are comparatively hard at 9 on the Mohs scale of hardness, second only to Diamonds with a hardness of 10. They are excellent stones to set in jewellery due to their ability to resist scratches as well as any chemical attack. It is not advisable, however, to put them in an ultrasonic or steam cleaner, especially if stone is fractured or has been lead-glass filled.
Rubies are a historic gem, considered to be the gem of royalty. Whether you appreciate them for their colour, multitude of inclusions, or their stature within the gem community, they are a worthy addition to anyone’s jewellery box, especially if you were born in July.
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Cover image: Crystal and Feather Inclusion. Image by Pat Daly.
Did you think this might be emerald? You certainly wouldn't be the first to confuse the vibrant green of dioptase with the more recognisable, jewellery-lover's gemstone. Here, Gem-A assistant gemmology tutor, Dr Juliette Hibou FGA, explains more about the history, properties and origins of dioptase, an unusual collector's gem.