Synthetic Corundum: Various Types Synthetic Process
Ruby was the first major gemstone to be synthesized, the first experiments taking place in the mid-nineteenth century. Today Synthetic corundum of all colours is manufactured by a variety of processes, for both gem and industrial use. Star corundum has been synthesized since the late 1940s.
Melt-Growth Processes
Verneuil (Flame-Fusion) Process Various manufacturers (all colours, including six-rayed stars).
1. Curved growth lines (thin striae or bands), seen best at roughly 90° to the boule’s length. These curved lines are not concentric.
2. Gas bubbles, round or elongated at 90° to direction of growth lines, from pinpoints to large distorted doughnut-shaped spheres or highly irregular worm-like distortions. They are usually distributed in clouds which follow the curved growth structure of the boule. In blue stone, the gas bubbles may show concentrations of blue colour around them.
3. (a) Polysynthetic twinning along the c-axis (‘Plato lines’), seen between crossed polars, most effectively when the specimen is immersed, in sets of one, two or three directions.
(b) Polysynthetic twinning along the rhombohedron, sometimes with accompanying boehmite needles identical to natural corundum.
4. Induced fingerprints and feathers entirely similar to natural corundum.
5. Traces of the seed rod or seed crystal. Found at the base of the boule and featuring frosted surfaces at the seed junction.
6. Some varieties may show useful UV fluorescence. The V-doped colour change type shows a diagnostic line at 475 nm. The full Fe spectrum (451, 460, 470 nm) is not seen.
7. Irregular colour distribution and rounded facet junctions of blue varieties,\ in particular, may cause confusion with surface diffusion-treated corundums.
8. Dense clouds of extremely fine, exsolved rutile silk in star material. The clouds do not show the angular zoning patterns that are common to natural stones. Instead, they may display curving bands (which are sometimes concentric).
Czochralski (Pulling) Process
Inamori [Japan] (red, orange and red star), Novosibirsk [Russia] (red) and others (red, blue and colourless).
1. Extremely fine and narrow curved growth lines (curved striae), which may be concentric.
2. Gas bubbles of various sizes, shapes and orientations.
3. Faint ‘smoke-like’ or ‘rain-like’ wisps of tiny particles, probably representing remnants of a flux used to aid melting during growth.
4. Small unidentified black prismatic crystals in groups (Inamori).
5. Extremely fine exsolved clouds of what may be rutile silk in the red star material (Inamori).
Floating-Zone Process
Hattori Seiko (red, orange, blue), Novosibirsk (red).
1. Gas bubbles of various sizes, shapes and orientations.
2. Irregular colour swirls.
3. Secondary lamellar glide twinning along the rhombohedron faces. Twin planes may intersect at 87 and 93, and may also show long white needles (boehmite?) at the junctions of intersecting twinning planes. Best seen between crossed polars while immersed.
4. Rectilinear parting.
Solution-Growth Processes
Flux Process
Chatham (red, orange, blue), Kashan (red), Ramaura (red), Knischka (red), Lechleitner (overgrowths in various colours), Novosibirsk (red), Douros (red).
1. Primary flux-filled negative crystals, often only partially filled (twophase) and featuring a characteristic crazed surface appearance. Primary flux in Ramaura stones often has a yellow-orange colour and may show distinct growth striation on flux surfaces, open cavities and crystal faces which mirror the colour zoning.
2. Secondary flux-filled fingerprints, feathers, etc.
3. Tiny flux particles, often arranged in streamer or comet-like patterns (such as ‘rain’ in Kashan stones).
4. Platinum plates, flakes, crystals, needles, etc. (Chatham, Knischka); black (platinum-rich?) growth planes (Chatham only, especially along the seed crystal).
5. Very small, oriented, exsolved silk-like needles and/or particles in zoned clouds (Chatham and Knischka).
6. Polysynthetic and growth twinning in various orientations, but without the boehmite needles often present in the natural, Verneuil and Seiko (floating zone) synthetics.
7. Straight growth lines running parallel to crystal faces and meeting at 11 specific angles. Unusual growth-line boundaries (Ramaura, Douros [Greece]).
8. Chatham: rounded transparent crystals of low relief (possibly chrysoberyl).
9. Seed crystal, generally with trapped flux on the boundary. The boundary may be difficult to see in the Lechleitner overgrowth.
10. Knischka: primary negative crystals which often display a two-phase filling. These may be bipyramidal or irregular in shape and sometimes are surrounded by irregular bluish white clouds.
Hydrothermal Process
Novosibirsk only (red).
1. Extremely strong growth zoning (graining) parallel to the basal plane.
2. Small, highly reflective crystals of a gold colour, consisting of copper alloys.
3. Secondary healed fractures (fingerprints) believed to be liquid filled. Simulants and Imitations Although there are a number of materials which can have a similar appearance to ruby and sapphire (such as certain spinels, garnets, benitoite, etc.), all are easily separated by reference to the properties, such as RI and SG.
More dangerous are assembled stones, generally consisting of one or more parts of natural corundum, attached to other materials (generally synthetic corundum). They may show natural inclusions and colours because they are part natural. The key to identifying any assembled stone is to locate the separation plane where the stone is joined together. It must be a distinct join, completely unbroken around the entire stone. It is usually on the girdle, but may be on the pavilion or even the crown. The glue layer often shows curved brush strokes and flat gas bubbles. Since the glue’s RI is quite different from corundum, it will stand out in high relief when the gem is immersed in di-iodomethane. Various kinds of assembled star stones are also possible. One of the most deceptive is simply taking a white star sapphire and coating it with red plastic.