Ruby

This red variety of corundum occurs as bright red, purplish or brownish- red and deep pinkish- red colors. Ruby is the birthstone for July.
One sixteenth-century writer wrote of ruby that ruby “gave control of the passions, drove out evil thoughts, secured possessions to their rightful owner, reconciled quarrels, brought peace and concord and also preserved bodily strength and health.”
In the past most red stones including ruby, spinel and garnet showing a good hardness and luster were often called ‘ruby’. As our knowledge of gems improved these other materials became better recognized in their own right, but misidentification is still common.

Common treatments

Ruby is heated to improve the quality of its color. Heating can, for example, remove the brown tones from Thai rubies and the blue tones from stones from Vietnam or Mong Hsu.
As a result of the heat treatment process, fractures and cavities may be filled with an artificial material. This material is an amorphous mixture of the molten powdered flux in which the rubies are placed for heating, plus material present in and around the original rough ruby. These glassy residues may be evident on the polished surface of the faceted gemstones as areas of lower luster in contrast to the higher luster of the surrounding host ruby. These glassy residues within the healed fractures can have the appearance of flux-filled fractures seen in flux melt synthetic rubies. This glass filling of rubies is also done intentionally to improve the appearance of the stone by hiding fissures and filling cavities and is also carried out using lead glass. This filler gives blue flashes as the stone is rotated and moved and gas bubbles may be trapped in the glass filler.

Red oil may also be used to reduce the visibility of fractures and to improve the color of a ruby.

Localities and inclusions

Ruby is found in commercial quantities in many locations:

Locality	Comments and inclusions
Myanmar (Burma)	Fine- quality rubies of good color are found in the Mogok district. These stones commonly contain short, fine retile needle inclusions known as silk. A variety of other mineral inclusions are also seen, particularly corundum, calcite, dolomite, spinel, zircon, garnet, apatite, graphite, pargasite and yellowish sphene crystals. Inclusions may be well-formed, or corroded, rounded crystals. The colour is often in swiris which have the appearance of treacle. Intersecting twinning planes are often seen. Rubies from Mong Hsu show central blue ‘cores’ in their untreated state, and whitish clouds of rutile; these stones are usually of a lower quality than those from the Mogok district.
Pakistan	Similar to those from Myanmar. Good colour, but clarity generally poor so mostly cut as cabochons.
Afghanistan	Usually only in small sizes, rarely producing stones in excess of 2 ct. Stones often contain blue patches similar to Vietnamese stones; also calcite, mica and rutile inclusions.
Thailand (Siam)	These rubies are naturally darker or more brownish-red than Myanmar stones. They often have irregularly-shaped fluid inclusions with dark crystals at their centres. Intersecting twin planes are also often seen. In heat-treated stones the colour can be very similar to that of Myanmar stones. Fluorescence is weak, due to iron content.
Sri Lanka (Ceylon)	Shades of red and pink. Stones often contain long coarse rutile needles, biotite mica, pyrite, metamict zircon grains with tension haloes, pronounced hexagonal colour zoning elongate negative crystals or cavities containing fluid and/ or crystals.
Tanzania	Stones from Logido are often of fine colour and are found in a bright green chrome zoisite rock. This combination of ruby and green zoisite is used as an ornamental stone. The ruby is usually cut as cabochons. Facetable material occurs in the Umba valley often showing twinning planes and apatite crystals. Star material is found at Morogoro.
Vietnam	Fine colour, good clarity, similar to Myanmar rubies. Much material contains blue patches, which may be removed or modified by heating.

Rubies are also found in many other localities including Australia, Kenya, Namibia, Madagascar, India, USA, Russia, China and Nepal.
Although these inclusions are frequently seen in gemstones from the areas indicated, they also occur in stones from other localities.

Material with a similar appearance

Red glass, red tourmaline (sometimes called rubellite in the trade), almandinepyrope garnet-topped doublet, and corundum-corundum doublet.

Tests

Dichroscope- Moderate to weak, red and oranges-red and dichotic.
Polaris cope and refract meter – Spinel and garnet are isotropic; the polariscope will reveal the singly refractive nature of spinel, but almandine usually shows anomalous extinction effects. If faceted, ruby is readily distinguished by its RI and birefringence.
CCF— Rubies, pink and purple sapphires contain chromium and therefore emit a red glow when viewed through a CCF as will most spinel; almandine will appear dark.
UV — They also fluoresce red in UV light. Natural rubies and purple sapphires often contain traces of iron, and this may sometimes mask their fluorescence.
Spectrum— The colour is caused by the presence of chromium, and the full diagnostic spectrum consists of a double line plus two weaker lines in the red, general absorption of the yellow and green, three fine lines in the blue, and general absorption of the violet. With a hand spectroscope only two of the lines in the blue are normally seen. A bright emission line may be seen in the red part of the spectrum. The spectroscope is useful to help distinguish red stones, particularly if they are small and set in jewellery. The garnet and ruby can be distinguished by their absorption spectra. The absorption spectrum of red spinel resembles that of ruby, but there are significant differences.

Quality Factors

Three important quality factors that affect value are colour, clarity and quality of cut. The highest value stones have a good rich colour, they are relatively free from inclusions and have a bright, lively appearance. In the series below, the stones are in order of value per carat, with the highest value on the left.