Know your Ruby

by Erum

The first clue to a true ruby's identity will obviously be its color. The name ruby is derived from the Latin 'ruber' meaning red. Corundum in its pure form is virtually colorless; the color red is induced due to the presence of impurities in the form of trace elements chromium or iron within the crystal structure of the gemstone. Ruby may range in color from red to purplish- or brownish red and deep pinkish-red. The most desired color for ruby is a deep red tinged with blue known traditionally as 'pigeon's blood' red though such terms are used increasingly rarely nowadays.

Know your Ruby

Ruby belongs to the gem family Corundum, the other member being Sapphire. Apart from emeralds, rubies are the most popular stones along with their cousin the sapphire. These three ruby, emerald and sapphire are also known as the 'Big three' and counted as precious stones (as opposed to semi-precious) no matter what their quality or caratage (weight).

Raw ruby

The first clue to a true ruby's identity will obviously be its color. The name ruby is derived from the Latin 'ruber' meaning red. Corundum in its pure form is virtually colorless; the color red is induced due to the presence of impurities in the form of trace elements chromium or iron within the crystal structure of the gemstone. Ruby may range in color from red to purplish- or brownish red and deep pinkish-red. The most desired color for ruby is a deep red tinged with blue known traditionally as 'pigeon's blood' red though such terms are used increasingly rarely nowadays.

Ruby

Second comes its heft- or weight proportionate to its size. Since corundum is one of the densest substances next to diamond (corundum's specific gravity being close to 3.8 or sometimes even 4) it is relatively heavier to the feel than its common look-alikes. Experienced personnel can estimate the heft of a stone and gain a clue to the identity of a gemstone. However this works only if the stone is out of it's setting of a big enough size. Smaller stones cannot be guessed at in this manner nor those that are set in jewelry.

Next to red diamonds rubies are probably the most expensive gems in the world in sizes above three carats. Transparent gem quality ruby is almost always faceted with lower clarities being cut en cabochon or as beads or are carved sometimes.

Ruby stone

The most important clues to a stones identity are its characteristic inclusions. These are described in detail below for both natural and synthetic rubies.

Occurrence

Ruby is found in commercial quantities in many locations around the globe. Some of the most important ones are Myanmar (Burma), Vietnam, Pakistan, Afghanistan, Thailand, Cambodia, Sri Lanka and Tanzania.

African stones make up a fairly large percentage of stones available on the market today. However Thailand still remains an important cutting and trading center for these stones despite being a relatively small producer of the stones.

Ruby is also found at other localities such as Australia, Kenya, Namibia, Madagascar, India, Nepal, China, Russia and the USA.

ruby diamond gold ring

Natural Inclusions

Most rubies will have the following inclusions regardless of the locality. Oriented needle-like inclusions known as 'silk' and slightly larger parallel inclusions- rutile needles. Healed fractures-these are liquid-filled residual cavities in a fingerprint-like healed fracture. Hexagonal growth zones are also a characteristic inclusion of corundum (both ruby and sapphire display this).

The table given below explains the various inclusions found in ruby from different locations although not all stones may necessarily display them.

Locality Comments and inclusions
Myanmar (Burma) from the Mogok district

The Mogok district is famous for fine-quality stones of good color. The stones commonly contain silk-short, fine rutile needle inclusions. Mineral inclusions are also observed such as corundum, calcite, dolomite, spinel, zircon, garnet, apatite, graphite, pargasite and yellowish sphene crystals.

Inclusions may be well-formed or corroded, rounded crystals. The color is often in swirls (appearing like treacle) and intersecting twinning planes are often seen.

 

Myanmar (Burma) from the Mong Hsu district Rubies from this district show central blue ‘cores’ when untreated and whitish clouds of rutile. The stones may be heat-treated to get rid of the blue color. The gems from this locality are generally of a lower quality than those from Mogok.
Pakistan Rubies from Pakistan are generally similar to those from Myanmar. Whereas they have a good color, clarity is generally poor therefore they are cut as cabochons.
Afghanistan The produce from this country is usually small in size- rarely above 2 ct. They contain blue patches similar to Vietnamese stones as well as calcite, rutile and mica inclusions.
Thailand (Siam) Rubies from Thailand are naturally darker or more brownish-red than Myanmar stones. Irregularly-shaped, fluid inclusions with dark crystals at their centers are usual as are intersecting twin-planes. When heat-treated the color can be every similar to Myanmar stones.
Sri Lanka (Ceylon) These gems range in color from red to pink. They often contain long coarse rutile needles, biotite, pyrite and metamict zircon grains with tension haloes.
Tanzania Rubies found at Mount Longido are often display a fine color and are found in bright green chrome zoisite rock. This combination of ruby and green zoisite stone is used as an ornamental stone. Ruby is cut as cabochons. Facetable material occurs in the Umba valley and shows twinning planes and apatite crystals.
Vietnam Fine color, good clarity, similar to Myanmar rubies. Much material contains blue patches, which may be removed or modified by heating.
Ruby necklace

Synthetic Corundum Inclusions

Corundum (both ruby and sapphire) is synthesized in laboratories by various methods. Each of these methods produces stones that have characteristic inclusions, some of these are listed below.

Method Inclusions
Verneuil flame-fusion Curved growth lines, gas-bubbles, induced fractures and healed fractures.
Czochralski pulled These stones are relatively clean but small gas bubbles may be seen.
Floating zone Seiko and Novosibirsk Gas bubbles and irregular color swirls.

Flux-melt:

Chatham, Kashan Ramaura, Knischka Novosibirsk, Douros 
Flux-filled cavities and healed fractures; tiny flux particles arranged in streamer- or comet-like patterns; platinum platelets; angular growth zoning similar to zoning observed in natural stones.

Flux-melt:  

Lechleitner overgrowth 
Seed crystals, generally with trapped flux on the boundary but rarely found.
Hydrothermal Wavy growth structure, feathers.

Treatment

Most commercial qualities of ruby and sapphire both are heat-treated to improve color. Even synthetic ruby (synthesized by the Verneuil method) is heat-treated to diffuse the striations. 

Removing brown tones from Thai rubies and blue cores and patches from Vietnam and Some ruby may be surface diffusion treated to add color to poor and colorless material. Rubies may be treated with red oil which enters fractures on improves color and clarity. Fractures, cavities and fissures in ruby may be filled with high lead content glass (more on this topic in the following pages). 

Detection of treatment

Color zoning lines are commonly poorly defined in heat-treated stones. Other characteristics include features like cross-hatched color zoning and sharply defined cloudy layers or patches. These appear whitish due to concentrations of sub-microscopic inclusions. 

Star ruby

Natural untreated star stones with good color and sharp stars are rare. Generally a six-rayed star is seen, rarely a 12-rayed one. Natural star stones are usually distinguished from synthetics by their coarser rutile needles.

star ruby

 
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