Rubies, and their close cousins, sapphires, are members of a mineral species called corundum. In gemology, a “species” is a mineral that has a definite chemical formula and a specific three-dimensional structure. Corundum is an aluminum oxide (Al2O3), and it has a regular crystalline structure that is formed by repeated patterns of arrangement at the atomic level. Crystalline minerals are classified into seven different crystal systems depending on the symmetry of their repeated atomic units. Corundum has a trigonal crystal system.
How Are Rubies Categorized?
A gem “variety” is a sub-group of a mineral species. Corundum is found in a number of different varieties, many of which are not as valuable or rare as ruby. Common milky-grey corundum, which is used as an abrasive, is called emery.
Different varieties of corundum are distinguished by characteristics of color, transparency , internal features, and optical phenomena. The key feature that defines a ruby is its fiery red color. Sapphire is another variety of corundum, and it comes in all colors except red. In essence, rubies are red sapphires, and sapphires are rubies that are not red.
Pure corundum is colorless, but most corundum contains minute amounts of chemical impurities called trace elements. Blue sapphires are colored by a combination of the trace elements iron and titanium. Iron and chromium together result in gold or orange sapphires, and when chromium is the only trace element, it creates a pink sapphire. Greater quantities of chromium give corundum the lustrous red of a ruby.
Although ruby is defined by its red color, there is no universal agreement on what characteristics define the color. Each culture has its own perspective on what constitutes “red.” In much of Asia, the color red—and hence ruby—encompasses shades Westerners consider to be pink. In fact, some Asian languages do not have separate words for pink and red. However, while pink corundum may be called ruby in some parts of the world, North Americans and Europeans restrict the term “ruby” to corundum that possess a medium to very dark red color.
Corundum is given a score of 9 on the Mohs Hardness Scale which assesses the durability of various gemstones. On the scale, each mineral can scratch the ones below it. Rubies and sapphires are second in hardness only to diamonds, making them both excellent gemstone choices for everyday jewelry.
Natural Features of Rubies
An unusual characteristic of ruby is its ability to fluoresce red in daylight. Fluorescence refers to an object’s ability to emit visible light when it is exposed to radiant energy. Fluorescence can distinguish ruby from other gemstones and it can also aid in determining their origin. Rubies that form in marble deposits, such as those from Myanmar, typically have this desirable characteristic. On the other hand, rubies from Thailand and Cambodia are formed in iron-rich basalt and will not fluoresce. Although these rubies may have an exceptional hue , they do not exhibit the supercharged red glow of those with fluorescence.
As famed sapphire and ruby gemologist Richard W. Hughes writes:
“When a ruby is put into daylight, certain electrons are excited to higher orbitals, producing absorption of the corresponding wavelengths. But instead of falling straight back to the ground state, the electrons fall in steps…In the case of ruby, some emissions fall into the red. This is what makes ruby so special; not only does it possess a red body color, but that red body color is supercharged by red fluorescence. This is what led the ancients to believe ruby had a fire burning inside.”
Some rubies are also treasured for a special optical phenomenon called asterism. Asterism refers to the stars that can be seen in some cabochon-cut rubies. Asterism is caused by tiny mineral inclusions within the ruby called silk. Most star rubies have six-rayed stars, but very rarely, twelve-ray stars also occur. Star rubies have been treasured as magical stones and amulets for many centuries.
With an understanding of how ruby crystals are formed, it is time to dive into a bigger discussion about the role that the 4Cs of Quality serve in judging the value and quality of a ruby.