Ruby and sapphires
1. NATURAL CORUNDUM
1.1 Color, tone, and hue in natural corundum.
Natural corundum can be found in all colors in nature with a wide variety of hue, tone, and saturation (according to the GIA grading system for colored gemstones) (picture 1.1.1). The color is determined with hue precision, for example, red-violet, violet, violet-blue, etc. Tone refers to a gem’s relative lightness or darkness of color, for example extremely light, very light, light. The tone scale is divided into 11 grades, 0 to 10, with 10 being black. Saturation refers to the intensity of color and is the degree to which color departs from a neutral (gray) sensation.
The red corundum with a color tone of over 5 is called ruby, and corundum of all other colors – sapphire (white sapphire, yellow sapphire, orange sapphire, pink sapphire, blue sapphire, green sapphire, etc.).
Picture 1.1.1 Different colors in natural corundum
The blue color tones from 5 to 9 for natural jewelry blue sapphires are shown in picture 1.1.2 from left to right. Most desired blue color tone for corundum is 8. When defining the color and color tone, the stones should be placed on a clean white paper with the pavilion facets up, as the dispersion from the crown facets can mislead us.
Picture 1.1.2. Blue color tones from 5 to 9 in natural jewelry blue sapphires
Ruby may exhibit one of a few possible secondary hues – pink, purple and orange. Most desirable and expensive are the rubies with light purple hue and tone 8. Their color is known as “pigeon blood” (picture 1.1.3).
Picture 1.1.3. Pigeon blood ruby.
1.2. Growth lines in natural corundum.
The growth lines (picture 1.2.1) in the natural crystals (including corundum) are straight and follow the directions of the crystal flat faces of the raw material before the faceting. Crystal morphology (crystal shape) is closely related to crystal growth. The internal regularity in crystals (atoms are arranged in an orderly, repetitive array) often shows itself on the outside of the crystal, as flat faces meeting at sharp edges and pointed corners. The growth lines can only be observed under a microscope with appropriate lighting.
Picture 1.2.1 Growth lines in natural sapphire
2. TREATMENT (ENHANCEMENT) OF CORUNDUM
Various techniques are known to improve the appearance of natural or synthetic corundum. Heat treatment with or without a chemical agent is accepted as a common practice. Ruby and sapphire can be heat treated with the following main purposes:
– to improve, add, or remove color
– to improve or remove asterism
– to improve clarity by removing silk.
2.1. Diffusion treatment of blue sapphires
The diffusion treatment involves the addition of color-causing chemicals during heat treatment. It results in a thin layer (about a millimeter) of color at the surface of colorless or light-colored sapphires. This process is done to create the star effect on the gemstones also and these gemstones are then called a diffused star sapphires. The new attractive blue color is stable for routine cleaning procedures. Polishing or recutting would take off this surface treatment. When heated, the optical effect may disappear. The optical properties (refractive index and birefringence) and specific gravity of diffusion-treated stones are the same as those of natural-colored and heat-treated stones.
This method of treatment is very easy to detect when immersed gemstones in methylene iodide. The three stones used in my experiment are shown below: left – blue untreated sapphire from Sri Lanka, in the middle – faceted sapphire with diffusion treatment only on the crown, right – cabochon-cut sapphire with diffusion treatment in all directions of the stone (picture 2.1.1).
Picture 2.1.1. The three stones used in my experiment.
In methylene iodide, untreated by diffusion sapphire shows low relief (it is almost imperceptible) and the facet junctions are almost invisible. Faceted sapphire with diffusion treatment only on the crown shows greater relief due to color concentrations at facet junctions. Blotchiness of the diffused color layer may also be observed. Diffusion-treated cabochon-cut stone shows a very high relief and deep color concentrations in cracks and cavities (picture 2.1.2.). If this sapphire was cut with flat facets, the edges between them would be in contrast to the facets.
Picture 2.1.2. The same three stones from picture 2.1.1 immersed in methylene iodide.
When immersed in methylene iodide untreated by diffusion blue natural sapphires can show color zoning and inclusions (picture 2.1.3 and picture 2.1.4).
Picture 2.1.3. Untreated by diffusion blue natural sapphires.
Picture 2.1.4. The same three stones from picture 2.1.3 immersed in methylene iodide.
2.2. Heat treatment of ruby with flux (fracture healing/filling)
This method of treatment is applied to full of cracks rubies in order to improve their purity. Ruby is heated in the presence of borax, which penetrates the cracks in it and partially dissolves their walls. The borax is then extracted from the cracks and an amorphous filler is introduced into its place. The filler has a close to corundum refractive index, and after its hardening in the cracks, they become practically invisible. This treatment is permanent and irreversible.
This treatment can be detected by microscopic observation. The filler strongly reflects the light and, depending on the lighting mode, is most commonly seen in silver color or in very dark tones of red color (pictures 2.2.1, 2.2.2 and 2.2.3). Sometimes residual flux forms a halo of treatment around destroyed inclusions or secondary “fingerprint” inclusions (pictures 2.2.1 and 2.2.4).
Cleaning in an ultrasonic bath for such crystals is not recommended.
Picture 2.2.1. Heat treated with flux ruby from Mong Hsu, Myanmar – a halo of treatment around destroyed inclusion, x 120
Picture 2.2.2. Heat treated with flux ruby from Mong Hsu, Myanmar – filled cracks and a secondary “fingerprint” inclusion, x 75
Picture 2.2.3. Heat treated with flux ruby from Mong Hsu, Myanmar – the filler is observed with a strong reflecting ability, x 90
Picture 2.2.4. Heat treated with flux ruby from Mong Hsu, Myanmar – secondary “fingerprint” inclusion, x 150
In this method, the stone pavilion is covered with a thin film of a chemical substance in order to improve gemstone color (picture 2.3.1).
Picture 2.3.1. Residues of the coating film on the pavilion of a natural treated sapphire.
2.4. Doudlets and triplets with corundum
They are made of two or three pieces that were stuck to each other. Most often the crown is made of pale corundum, and under it, glass with saturated red or blue color is glued. In triplets, the middle part is made usually from deeply colored resin.
Doublets and triplets can be distinguished by the gas bubbles as a result of the bonding (picture 2.4.1 and 2.4.2), different inclusions and physical characteristics of the crown and the pavilion, along with the line of bonding if the stones are loose.
Picture 2.4.1. Gas bubbles in a doublet with ruby
Picture 2.4.2. Gas bubbles in a doublet with ruby
3. Synthetic corundum
Synthetic corundums have the same chemical composition as natural corundums, identical color, hardness, refractive index, birefringence, and specific gravity. Their synthetic origin is recognizable by microscopic studies of inclusions (do not contain natural inclusions) and some growth features. They can also be identified using spectroscopic methods of investigation to prove the presence of flux traces in their chemical composition.
There are a number of different ways to grow corundum. Most widely used are the following three: Flame Fusion, known as the method of Verneuil (picture 3.1 and 3.2); Czochralski pulling process and Flux growth process.
Picture 3.1. Synthetic ruby produced by the method of Verneuil
Picture 3.2. Synthetic orange sapphire (method of Verneuil)
First two methods are technically almost identical. Flame Fusion and Pulled corundum are grown very fast by melting and subsequent cooling the constituents of corundum. They have no natural looking inclusions and are easily distinguished from natural corundum by the presence of curved growth lines (picture 3.3) and gas bubbles (picture 3.4). The gas bubbles in the Flame Fusion (Verneuil) corundum are two types – single with a subspherical shape usually occur in layers that follow the curved growth lines and elongated gas bubbles, named “torpedo”, perpendicularly located to the curved growth lines. Verneuil corundum grows into boules (the boule is formed in the shape of a tapered cylinder). The raw material obtained by the method of Czochralski has a form similar to the meniscus.
Picture 3.3. Curved growth lines in a Verneuil orange sapphire
Picture 3.4. Gas bubbles in Verneuil orange sapphire
Flux growth technique is a high-temperature flux method of growing synthetic corundum with spontaneous nucleation, which produces many different external crystal forms of the raw material. Some manufacturers use a seed of synthetic corundum to start the growth process.
The Ramaura flux corundum has many characteristics that closely correspond to those of natural crystals – straight growth lines, color zoning, inclusions, visual appearance. The most important means of distinguishing the Ramaura synthetic corundum from natural ones is provided by the inclusions present.
Inclusions in Ramaura synthetic corundum (The Ramaura synthetic ruby by Robert E. Kane, GEMS & GEMOLOGY, 1983):
- Flux inclusions – frequently appear orange-yellow, although they may also range from nearly colorless to white in the form of “fingerprints” that ranged from transparent to opaque, and near colorless to white, in low to high relief; drops or grains; wispy veils; parallel groups of voids filled with orange-yellow flux. They may give the appearance of minute two-phase-like inclusions but are completely solid;
- Fractures and healed fractures – they cannot be considered diagnostic of synthesis;
- “Comet tail” inclusions;
- Optically detectable inhomogeneities such as twinning, parting, structural defects, and growth phenomena such as “phantoms”;
- Angular or “hexagonal” color zoning;
- Thin metallic flakes of platinum from platinum crucibles.
Picture 3.5. Flux growth ruby