Our basic gem identification and approximate valuation method is free and is linked to the area of our
site that asks folk to answer a series of questions about their gemstone or
their piece of jewelry. Should the results seem promising we report back, follow up and
advise on what further action you may wish to take. It should be fun and there is no obligation or cost to us
no matter how many times it is used. It is configured for both android and large screens.
Here is the link:
Regarding our ratings method see under the "technical notes" heading.
CGG receives a varied and complex range of items for identification;
everything from colored stones and pearls, unusual carvings and jewelry,
to the latest in synthetics, and simulants, as well as treated gems such
as ruby, and sapphire.
Gem certification can cost between $10 and $300 + depending on the wants of
Should a customer only require a certification that a pink gemstone
is indeed a natural pink tourmaline only 2 or 3 tests would be necessary such as
optical character, specific gravity and refractive index.
These tests are relatively fast and inexpensive.
Should however, a yellow gemstone need to be tested to establish it is
an unheated, natural, untreated, yellow sapphire from Sri Lanka, then all
tests including laser induced breakdown spectroscopy would be required.
CGG would does not have such facilities.
Should you wish to find some basic information on your gem type please go here. We list over 50 gem types.
In our 'test yourself' section we have added a method to test the fire (dispersion) of a gemstone.
"Fire" refers to the rainbow-like flashes of color seen in cut stones.Fire is especially obvious in diamonds.
Where does it come from?
It is important to realize that the extent to which light is refracted (bent on passing into or out of the gem) is dependent upon the wavelength (color) of the light. Note that blue light is bent more than red light
The phenomenon of different amount of bending of different colored light is referred to as dispersion.
Dispersion is measured: dispersion = refractive index of violet - refractive index of red light.
Dispersion varies greatly with the mineral type. Lists of dispersion values are available online.
The fire of a gem is a consequence of the cut of the stone, coupled with its dispersion.
Our sunlight and shaded paper method will show a rainbow effect should the gem be reasonably faceted or cut and
the three alternatives of high, medium and low in the selection is not a particularly accurate method but
for the non-gemologist it may help to differentiate the gem type.
Experience can teach you to determine the fire just by holding the gem in your hand and moving it about.
Now lets get to the hard one: When the properties of a material (here a crystal)
vary with different crystallographic orientations, the material is said to be anisotropic.
Alternately, when the properties of a material are the same in all directions,
the material is said to be isotropic. The index of refraction depends on both
composition and crystal structure and can be calculated. Crystals are often
"Birefringence" also is the optical property of a material
having a refractive index that depends on the polarization and propagation direction of light.
Many crystals have some degree of birefringence or are anisotropic and these we call "pairs' as the two
resultant beams sustain a constant parity whereas a crystal with almost zero birefringence or is isotropic
(diamond, garnet) appear to have no "pairs". The sunlight and paper method is an elementary
method of determining this difference.
In our ratings system we rate an average gemstone of all mineral crystals in regard to
their availability or rarity. However with new discoveries this may change. Obviously
the best of the best gemstones with pure color, absolute purity and perfect cut are so
rare they do not come into normal calculations although they might be sometimes roughly
compared against the best of the best of each other. That is not useful unless you are
a millionaire collector or a museum. So who knows rarity? Wholesale gem markets do and
is where our experience is determined. These markets may be in different continents but
most are similar regarding a gemstones quality and availability.
First we rate the availability on a scale of 1-1000 with 1 being the rarest and 1000 the
Thereafter the ratings system using the exponentiation mathematical operation involving
two numbers, the base b and the exponent n. When n is a positive integer, exponentiation
corresponds to repeated multiplication of the base: that is, bn is the product of
multiplying n bases.
For instance in generally determining our pearl prices the n base
is 0.6 whereas b is the diameter of the pearl.
In other words an almost exponential curve where price increases with carat weight.
Our system only uses what we consider useful carat sizes for the general buyer.
Extra small and large sizes tend to distort the value curve and are generally not
In our 'Value your own Gem' pages elsewhere we do attach some variations regarding inclusions, clarity, windows and color variations
as a percentage decrease towards the final estimation and do all this without literally sighting the gemstone so folk must be aware of the
limitations of the system with a particular gem.