Gemstone Treatments & Enhancements: Part 6

The mere mention of radiation is enough to scare some people, driving them from the store counter to the nearest Geiger counter. However, although it is true that at this very moment, unsuspecting children are trying to sell hot blue topaz on the streets of Rio de Janeiro, tight regulations in North America and other parts of the world strictly regulate the importation and exportation of irradiated gemstones.

Like heat treatment, radiation causes profound changes in gemstones and while most are permanent and stable, others are not.

In this issue, we will look specifically at topaz, yellow and padparadscha sapphires, natural and lab-created quartz, Maxixe beryl, pink and red tourmaline, kunzite, and pearls and the characteristics that allow us to detect this enhancement technique.

Blue Topaz on White Preforms

Blue Topaz on White Preforms

Irradiated Blue Topaz
Perhaps the most commonly encountered irradiated gemstone is blue topaz. Although it does occur naturally, most natural blue topaz is very pale in colour and of little or no value.

Available in three shades: light blue, sometimes referred to as sky or sierra blue, medium blue, often referred to as Swiss blue, and a darker greenish-blue known as London blue, it is produced from white topaz, which is irradiated and then annealed. Three different types of radiation are used to produce the different colours, namely, gamma rays from a cobalt-60 source, for light blue, high-speed electrons, from a linear accelerator, for medium blue and high-energy neutrons, from a nuclear reactor for darker London blue stones.
Once irradiated the stones are annealed at between 200 to 250 degrees Celsius for two hours to remove the unwanted yellowish and brownish overtones caused by the brown colour centres formed during the radiation process. This also stabilizes the resultant blue colour.

Detection
It is generally assumed that all blue topaz has been irradiated and then heat treated, therefore no special provisions are made in the price guides other than the price differential between the different colours with light blue being the least expensive and London blue being the most expensive.

Although it is unlikely that the stones are still radioactive, due to the tight regulatory controls in North America, this can be tested using a Geiger counter or by placing the stones on a piece of photographic film. In the event they are still radioactive, the stones will darken or fog the film. This technique is used by workers in the nuclear industry who are required to wear film badges which are then sent to a laboratory, usually each month, to be developed just like regular photographs. This allows them to determine the level of radiation each worker has been exposed to.

Irradiated Yellow Sapphires
Through exposure to gamma rays, it is possible to impart a deep orangey-yellow colour in pale yellow sapphires. This causes the ferric iron oxide (Fe3+) to convert to ferrous iron oxide (Fe2+)

Yellow Sapphire

Yellow Sapphire

(Nassau 1984). This treatment is unstable and must be disclosed since stones will fade when exposed to strong sunlight or when heat is applied.
Detection
Suspected stones can be subjected to a fade test but this has to be approved by the client prior to any tests being conducted. Even though it is difficult to detect, the 450nm absorption line will be absent in irradiated stones, but since this is very faint in untreated stones, conclusive testing can only be done using a spectrophotometer.

Irradiated Padparadscha Sapphires
Pink sapphire can be transformed into padparadscha sapphire when subjected to radiation. Similar to irradiated yellow sapphire this causes the ferric iron oxide (Fe3+) to convert to ferrous iron oxide (Fe2+) when chromium is present (Nassau 1984). This treatment is unstable and must be disclosed since stones will fade when exposed to strong sunlight or when heat is applied.

Detection
Similar to irradiated yellow sapphire, stones can be subjected to a fade test but this has to be approved by the client prior to any tests being conducted. Due to the high values commanded by fine quality padparadscha sapphires, all suspicious stones should be referred to a recognized laboratory.

Irradiated Lab-Created or Natural Quartz
A purplish colouration is produced by subjecting hydrothermally grown quartz or colourless, yellow or pale green natural quartz with trace amounts of Fe3+ (ferric oxide) to gamma rays. The radiation causes the Fe3+ to convert to Fe4+.

Detection
The same characteristics that identify lab-created quartz can be used to detect irradiated lab-created amethyst quartz. These include the presence or absence of interference bands, breadcrumb like inclusions and tiny fluid inclusions, which can sometimes be confused with the typical tiger strip inclusions often seen in natural amethyst. However natural quartz treated with radiation is difficult to detect unless it is tested by a recognized laboratory.

Irradiated Maxixe Beryl
Around 1970, a dark-blue beryl started to appear in the market resembling natural Maxixe beryl. The original material was discovered in 1917 in the Maxixe mine of the Piaui area south of Arassuahy in Minas Gerais, Brazil.

Natural Maxixe beryl derives its colour from exposure to natural radiation in the ground, which creates a colour centre. However, this colour is considered to be extremely unstable and often fades when exposed to strong sunlight or temperatures over 100 degrees Celsius.

The new material consisted of yellow and green beryl that had been treated with radiation to turn it a dark-blue. This was due to the CO32- converting to CO3- or NO32-, which in turn converted to NO3- (Andersson 1979). Like the natural material the colour was also unstable tending to fade when exposed to strong sunlight and heat.

Detection
All dark-blue beryl should be treated with suspicion and referred to a recognized laboratory. Although it is possible to subject the stones to a fade test, with the consent of the owner, this will still not determine whether or not the colouration is due to natural or artificial radiation.

Red Tourmaline

Red Tourmaline

Irradiated Pink & Red Tourmaline
Enhanced pinks and reds are produced by exposure to gamma rays, which converts the Mn2+ to Mn3+ (Manning 1973). After treatment, some stones may still be radioactive and should be handled with care.

Detection
Unless the stones are still radioactive, colour enhancement through radiation cannot be proven. Unfortunately this technique is used to not only intensify the colour in pink and red stones but also to convert less valuable light pink, green and even olive grey-green stones to more intensely coloured pinks and reds.

Irradiated Kunzite
Discovered in 1902 and named after George Frederick Kunz, chief jeweller and noted mineralogist at Tiffany & Co at the time, kunzite derives its colour from small

Kunzite

Kunzite

trace amounts of manganese, and like amethyst quartz, will fade if exposed to strong sunlight or heat. However when irradiated the Mn4+ converts to Mn3+ (Petrov 1990) creating an unstable and usually radioactive stone that should be handled and stored with extreme care in a radiation-safe lead container.

Detection
Although rarely encountered, due to the residual radiation all stones should be tested with a Geiger counter or placed on photographic film (see blue topaz previous page).

Irradiated Pearls
Pearls are often colour enhanced using gamma radiation and radiation from a particle accelerator.

Detection
A thorough examination of the drill hole will reveal to what extent the colour permeates the pearl. In the case of black pearls, an absence of brownish-red fluorescence under long wave UV light indicates that the pearls have been irradiated. JBA

 

 

Geoffrey_M_Dominy_Geoff_v3Geoffrey Dominy is an author and gemologist now living in Lima, Peru. For four years, he was the senior jewelry appraiser on the CBC Canadian Antiques Roadshow. He is a Fellow of the Gemmological Association and Gem Testing Laboratory of Great Britain, passing his diploma examinations with distinction. He has been appraising, lecturing and teaching since 1987 and was a contributing author for both the 5th & 6th Editions of Robert Webster’s ‘Gems’. He has just released the first digitized gemmology book entitled ‘The Handbook of Gemmology’ that features the photography of Tino Hammid.

Tino’s photographs have appeared in countless books, major jewelry publications, and advertisements. Winner of two Jesse H. Neal awards from the Association of Business Publishers for his work with David Federman and Modern Jeweler, he has also photographed more than a hundred Jewellery sales catalogues for Christie’s Auction House. His business is located in Los Angeles, California.

Author:Jewelry Business Advisor