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Gem
Testing
The gemologist must identify gems nondestructively.
This limits the available testing methods to optical
and physical measurements, rather than scratching or
chemical analysis. Some features are so characteristic
of particular gems that visual identification can be
made with certainty.Color alone is not a good identification
criterion, for it can be misleading. Color merely helps
the gemologist limit the range of possi¬ble choices.
The eye is limited in the detail it can resolve, and
inclusions in gems are often diagnostic. A binocular
microscope that creates a stereoscopic image is therefore
widely used for examining gems.
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In
some cases particular inclusions can indicate the mine
from which a gem came. Synthetic gems have characteristic
internal features useful in identification. Verneuil-grown
synthetics display curved growth lines, never seen in
natural gems. Round bubbles are also diagnostic of synthetics
or glass. |
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Refractive
index is a useful parameter, usually measured with a device
called the refract meter. The eye can judge refractive power
to some degree, but cutting disguises this property in many
cases.
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DIAMONDS,
GEMSTONES & CRYSTALS
FACTS & FIGURES |
| Gem
Testing
Specific
gravity is an important gem property, usually
measured by means of so-called heavy liquids.
The specific gravity of a gem is determined by
seeing whether it will sink, float up, or remain
stationary in a liquid with a known specific gravity.
Fluorescence is diagnostic of many synthetic gems,
such as Chatham emeralds and various synthetic
rubies. Doublets and glasses may also fluoresce,
revealing their origin or manufacturing method.Pleochroism
in gems is detected with devices called the Polaris
cope and the dichroscope. The former instrument
also measures double refraction. These devices
are simply constructed from sheets of polariz¬ing
material such as are found in sunglasses.The spectroscope
is useful in identification, because it reveals
the so-called absorption spectrum of a gem.
Other gem-testing tools include solvents for removing
coatings and dyes, a hot-needle for testing amber
and turquoise, dilute acids, and a simple magnet.
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Lapidary
Hobby
Lapidary, the art of gem cutting, is one of the
fastest-growing hobbies in the world. In the United
States some hobby magazines have paid circulations
numbering tens of thousands. Dozens of books have
been published on all aspects of the lapidary
craft, covering everything from grinding cabochons
to setting gems in handmade jewelry.
In the United States mineral and gem clubs are
present in nearly all major cities. A large percentage
of these clubs are organized in a Federation System,
with various regional Federations affiliated together
as the AFMS, American Federation of Mineralogical
Societies, which celebrated its 25th Anniversary
in 1972. |
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The
member clubs have many activities, including field trips,
sponsored lectures, tours, auctions, spe¬cial exhibits
and community programs, and swapping sessions. Some clubs
have sufficient resources to host shows, where invited dealers
sell their wares at booths. Such shows are major local events
and offer the gem enthusiast an opportunity to see many cut
stones, usually at prices lower than those charged in retail
shops. The Lapidary journal contains in every issue a major
calendar listing local shows throughout the United States
and Canada.
Mineral
and gem shows are also places to see lapidary equipment, rough
material, supplies, books, and hobby accessories and tools.
At many shows there are demonstrations of specific techniques,
such as carving or faceting. In addition there are usually
many different kinds of exhibits, including minerals, gems,
fossils, educational displays, and specialty crafts.
The
April issue of the Lapidary Journal also lists most of the
clubs in the United States and Canada, including the name
of the secretary, address, and where and when the club meets.
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