The various enhancements used today include:-
- Heat Treatment.
- Surface Modifications.
- Composite or Assembled Stones.
Heat treatment is mostly applied to low-grade gems in order to enhance their clarity or color. Depending on the gemstone, this process may involve low or high-temperature heating with the limits being the melting point of the material.
Is defined as the controlled heating of certain stones in order to effect a change of color. Certain stones are amenable to such heating and are considered quite stable while others are not.
Since no two stones are alike, each possesses the potential to react differently. The Stones in which the heat treatment is considered stable are considered commercially acceptable.
Detection, for the most part, is difficult. In some cases, identification is based on the fact that certain colors are rarely found in nature and are therefore considered to be enhanced.
In other instances, the procedure is so widespread (i.e. Citrine Quartz) that it is automatically assumed to be treated.
Example: Blue Zircon which is produced by heat treating Brown Zircon.
Irradiation is followed by the heating process to produce a better or new color for the gemstone. With the help of irradiation, the gemstone gets its new and beautiful shade and color.
Confined to a smaller group of gemstones, this process can occur in a variety of ways:-
- Radium Treatment
- Electromagnetic bombardment
- Neutron Radiation
- Electron Radiation
Caused by exposure to alpha particles emitted by radium salts.
Stones are radioactive and will fog a photographic plate if left in contact for several hours in a lightproof box. The coloration (green in the case of diamond) is on the surface only and is normally detectable upon close examination.
Electro Magnetic Bombardment
Bombardment of particles accelerated to enormous speeds in a cyclotron. In diamonds, the color produced varies from green to black depending on the length of the treatment.
Detected by immersion in a highly refractive liquid since the color is on the surface only. Yellow diamonds produced by further heating show a characteristic absorption band not seen in natural yellow diamonds.
Treated stones will invariably show a ring around the girdle or an umbrella effect around the culet.
Irradiation with neutrons from an atomic reactor. Coloration is through the entire stone rather than on the surface only.
Brown and yellow diamonds produced by this method have a characteristic absorption band of 594nm but green diamonds is undetectable except for a variance in color compared to the naturally occurring stone.
Coloration produced by using an accelerator. Coloration in the case of diamonds ranges from a pale blue to a blue-green but is on the surface only. The color resembles a rare type of diamond known as Type LLB diamonds.
Detectable by the fact that treated diamonds are non-conductors of electricity while natural blue diamonds are semiconductors. Surface coloration is evident when the stones are immersed in a highly refractive liquid.
Most irradiated stones are considered stable, but there are more cases of instability than in heat treated stones. Again commercial acceptance hinges on the degree of stability while public disclosure is open for debate.
These can include any of the following:-
- Impregnated with colorless or colored oils, waxes or plastics.
- General dyeing
- In most cases, the stones are merely used to imitate more expensive stones (i.e. Chalcedony) and can be identified by variances in physical properties and by microscopic examination.
- In the case of dyed green Jadeite, identification is possible by the variances in the absorption spectra and by its reddish or pinkish appearance under the Chelsea Filter.
- Some stones may fade under strong light.
- Immersion in an R.I liquid may reveal localization of color. (Should not be used for porous stones)
- Rubbing the material with a cloth or cotton swab previously soaked in acetone.
- Color rubs off on the thread used for beads.
Reasons for using impregnations
- Hide surface cracks.
- To stabilize material (i.e. Turquoise) or to protect it from acidic skin oils.
- Reduce porosity and improve color.
- Produce a hard coating over a roughly ground surface to give the impression of a polished surface.
- Provide protection for softer stones.
- Fill cracks thereby improving the colors.
- To imitate other stones.
Can be through the application of any of the following:-
- Paint or Varnish
- Foil backs
- Mirror backs
- Synthetic overgrowth.
Stones constructed of two or more pieces of material which may or may not be genuine. Composite stones can fall into one of the following categories:-
- True Doublets – consisting of two pieces of genuine stone (i.e. Sapphire/Sapphire)
- False Doublets – where the pavilion is normally glass or an inferior stone.
- Triplets – consisting of three pieces.
- Set in a liquid of the similar refractive index. In the case of a Soude Emerald, the colorless spinel crown and pavilion will disappear in methylene iodide leaving a disc of green gelatine.
- Joining a line visible under microscopic examination.
- Differences in luster.
- Bubbles where the two planes meet.
- Refractometer tests are useful for most crowns are either quartz, spinel or glass.
- Red “Halo” effect seen around the girdle of an Almandine Garnet doublet (in colors other than red) when placed table down on a piece of white paper.