Rubies (and the other corundum variety, sapphire) are an oxide written as Al2O3, formally called aluminum oxide. The difference between rubies and sapphires is the presence of trace elements, since pure corundum (white sapphire) is colorless. In rubies, traces of chromium (Cr) is what creates the red color. When there is not enough chromium (red) in the stone, it is not a ruby but a pink sapphire.
Rubies are a mineral, which is defined as a naturally occurring, inorganic element (a single element) or compound (multiple elements) with an organized atomic structure, chemical composition, and physical properties. There are seven crystal systems to categorize the structures of crystals, with corundum falling into the trigonal system.
This means rubies are doubly refractive (the crystal makes light split in two directions, showing double). It causes the stone in question to show different colors with different viewing angles (this is more visible in some stones than others). Since the rubies are doubly refractive, that means two colors. Even the most perfect rubies usually show a red and less-desirable orangy red, which makes the cutter’s job crucial.
From the top, U11385 shows a pinkish red, but when viewed straight through the side the color looks a little desaturated and orangey. The different colors showing from different directions is formally called pleochroism. It happens because rubies make the light split in different directions inside itself. In rubies, this is specifically called dichroism since the light is split into two directions. Some gems are trichroic, splitting light into three directions.
Rubies usually do not have very strong pleochroism, but it remains present and can be the difference between a vivid color and a dull red. In other words, the gem cutter’s paycheck.
Out of tons of mined rock only a few carats of ruby qualify as gem quality, and a lot more material (Over 97% of all rubies are heat-treated to better quality, many of which would not be gem-quality without treatment) that qualifies to be heat-treated to gem-quality. The best, clearest stones are faceted. Below that is cabochon-grade (A cabochan is a gem polished into a rounded form, and an easier cut on gem material than faceting) material, with the exception of rubies that exhibit phenomena like stars and on rare occasions a “cat’s eye” effect. These are evaluated by separate standards. Below that is carving-grade material ranging from partial transparency to completely opaque. The less transparent the ruby material is, the less rare it is.
When exposed to UV light (including the UV rays in sunlight), rubies fluoresce, or glow red. This is due to the presence of chromium (Cr). While chromium is what causes ruby red color, not all rubies glow red. Basalt hosted rubies, meaning high iron rubies from certain locations, do not fluoresce. The presence of iron inhibits this glow, which creates a visible difference between high iron rubies (basalt hosted) and low iron rubies (marble hosted). Like anything else there can be exceptions, though the visibility of fluorescence is dependent on the amount of iron.
Even pink sapphires will fluoresce red due to chromium content.
R2816 | left | medium | play | “Ruby ID: R2816 Weight: 1.69CT Origin: Myanmar”Phenomena are special effects present in certain stones. Rubies can show asterism, meaning a star. This occurs from “silk” according to the trade, which is more formally known as rutile needles. Rarely stars can be caused by needles of hematite instead. These needles are microscopic and very fine, reflecting light in specific directions. Rutile and hematite needles grow in three intersecting directions, which creates six visible arms.
Rutile and hematite needles grow in different directions, which means if both are present a 12-ray star is possible. However, this is very improbable in fine quality rubies since the stone would most likely be highly included and be a Black Star Sapphire instead. The more stuff that is in a stone, the less transparent it is. In rare cases a cat’s eye effect (formally called chatoyancy) is possible.
Corundum is also capable of changing color the way alexandrite does, but will never reach ruby red. Even if corundum could reach that color as a color-change stone, it would no longer qualify as a ruby. Instead it would be a color-change sapphire, even if the colors were ruby red and emerald green, since the dominant color of the stone is not just red.
The color range of these sapphires is usually blue to purple, rarely from green to reddish-brown.