Gem Identification

Physical observations from uncut gems

Hardness:

mohs hardness scale and other measurements. Although a somewhat destructive test, hardness values can be quite diagnostic.

Cleavage: 

preferential breakage along some surfaces e.g., mica. The planes along which minerals break are planes where atomic bonds are weaker.

Streak: 

color of powdered material when scraped across a ceramic tile. 

Note: Color and streak are not necessarily the same thing. For example, hematite has a black metallic luster, but streaks red.

Crystal Morphology: 

The external shape of crystals, reflects the crystal symmetry, and thus the atomic arrangement. e.g., hexagonal prism vs. octahedra. This is based on internal atomic order and can not be determined from faceted crystals!

Observations from cut and uncut gems

Color
Generally unreliable! It MAY be a clue (note the luster).


Specific Gravity

Specific gravity is measured by comparing the weight of the gem in air with its weight suspended in water
density = amount of mass per given volume: measured as grams per cubic centimeter.  Specific gravity varies with chemical composition and crystal structure type.


Refractive Index

These measurements are relatively straight forward for stones in settings.

RI determination using a refractometer:  because the critical angle is determined by the refractive index, measurement of the critical angle, specifically the angle at which total internal reflection occurs, allows determination of the refractive index.
 

 RI=sin (CA) * N 


(N is a known number)) and "CA" is the critical angle:

Refractometers are the equipment used to measure the critical angle.
How to measure the RI using a microscope.
 

Dispersion: 
The extent to which blue light is bent more than red light as it passes from air into a crystal (dispersion) can be measured and the value compared to tabulated values.


Pleochroism: 

A mineral whose color changes with its orientation is said to be pleochroic or to exhibit pleochroism. 

Specific minerals exhibit specific pleochroic colors. Cabochons of Cordierite exhibit this phenomena very clearly.


Other techniques (EPS 2 students are not required to be familiar with these): 

Spectroscopic techniques: X-ray images (e.g. of pearls) or X-ray Diffraction (used for the measurement of the atomic arrangement). Other techniques include Raman and Electron Microprobe. Spectroscopies revealing absorption details are especially important.

Other optical properties:

The behavior of light as it passes into a crystal depends upon the structure and chemistry of the mineral.

An important test involves the use of two polarizing filters that are oriented normal to one another.  The crystal is placed between the polaroid filters and the result is observed.

Note that in the absence of a crystal between the two sheets of polaroid, light is completely absorbed at the second sheet of polaroid. 

Optical behavior is subdivided into three types:

• Isotropic: material always remains dark under cross polarized light
• Anisotropic: light passes through the crystal as it is rotated under crossed polars.
• If, as it is rotated about one specific axis (only), the crystal remains dark, then the gem is termed "uniaxial."
• If two specific axes can be found where the crystal remains dark while rotated between two crossed polars, the crystal is termed "biaxial."
• Gems are classified firstly on isotropic, uniaxial, or biaxial categories, and then into more specific crystal systems.


Determination of optical properties allows a gem to be assigned into a group, each group is consistent with either one or a small subset of the possible crystal systems. This can often be used to distinguish between possible identifications. If you don't understand how this works, please look at these examples.

There are 6 different crystal systems characterized by different shaped unit cells.