Rubies and sapphires are from the same corundum family. Rubies are red-coloured corundum. Corundum of any colour other than red must be referred to as sapphires. The problem is determining what is ‘red’? Everyone perceives colours somewhat differently. So there must be a range that is acceptable. The second problem then is: what is an acceptable range? There is no consensus in the industry. Everyone pretty much agrees that the truest ‘red’ is the red referred to as ‘pigeon blood’ from the Burmese variety of red ruby.
As rubies command a higher per carat price than sapphires from the same corundum family, it is not unusual therefore to find vendors selling what are distinctly pinkish or purplish sapphires as rubies. Specimens that tend too far into the pinkish range or purplish range, however, can no longer be legitimately categorized as rubies and should more appropriately be referred to as either pink or purple sapphires. This is not to say that pink or purple sapphires are without value – they certainly are – but buyers should be aware that prices should be closer to the range for sapphire prices and not into the range for rubies.
Appearance to the naked eye:
Few buyers go into a retail stores with a loupe and gemological tools in tow. They rely on how appealing the gem is to them purely from a visual perspective. And this is also the case when you are out to dinner or in a social gathering – it is highly unlikely that the other guests are using sophisticated laboratory tools to admire your ring; instead they are relying on what they see with their naked eyes. That is why we want to spend a bit of time explaining what it is you can expect to see just by eyesight.
Red. Any colour other than red is more legitimately referred to as a sapphire and not a ruby. A range of ‘red’ is acceptable but do note there is no standardized or universally accepted range. As mentioned above, it is not uncommon for vendors to sell pinkish or purplish sapphires as rubies due to the fact that rubies command a higher per carat prices than sapphires can.
Anywhere from fully transparent, to translucent, to completely opaque. Generally speaking, the greater the transparency, the higher the price the specimen can command on the market.
Vitreous (glass-like) for transparent stones to greasy (wax-like or jade-like) for translucent and opaque stones.
White streaks; that can sometimes give the stone a marble-like mottling.
As with other stones from the corundum family; namely sapphires, rubies may present a strong pleochroic effect resulting in observing two different colours depending of angle of view or lighting conditions. The two colours are typically red and a lighter reddish tint tending towards pink.
Rubies, just like sapphires, can present a beautiful star-like formation referred to as asterism. The asterism effect may be either a six-point or twelve-point star.
Rubies are well-known for their inclusions and these can even be a visual confirmation that the stone is natural ruby rather than a laboratory-created synthetic product or some other simulant. Generally, the presence of visible inclusions will not necessarily negatively affect prices unless the inclusions are especially large or noticeably detract from the stone’s visual appearance and beauty.
- Other interesting facts:
- "Pigeon blood" red is the term used in the industry to denote pure red colour. Burma is especially known for producing much of the pigeon blood red specimens.
- Laboratory-created synthetic ruby material has been produced since the late 1800s for industrial purposes. One of the earliest applications was as bearings in wristwatches to reduce wear-and-tear at pivot points and extend the watch movement's lifespan.
- Rubies, like all corundum family gemstones are second only to diamond in hardness. It is this feature that makes them so valuable in industrial applications.
- Many pinkish or purplish stones marketed as rubies would more legitimately be referred to as pink or purple sapphires.
- Technical Details
- Refractive Index: lower reading in the 1.672 to 1.766 range; higher reading in the 1.774 to 1.778 range
- Birefringence: 0.008
- Optic Character:(U-) double refractive, uniaxial, negative
- Conoscope Image:cross-hairs
- Spectral Absorption: rubies are coloured by chromium. People familiar with spectroscopy will know to look for the tell-tale chromium line absorption pattern in the red end of the colour band. Some ruby species also contain iron, which presents its own distinctive absorption line in the blue end of the spectrum.
- Hardness (Mohs scale): 9.0 (second only to diamond)
- Specific gravity: 3.97 to 4.05 range. Average of 4.0
- Chelsea Filter Reaction: As most species of rubies contain iron, expect a dull reaction to the Chelsea filter. A bright reaction is evidence that the stone may be synthetic, but it is in no way diagnostic. Other tests would be required.
- Dichroscope Reaction: Red; plus pale red tending towards pink