Physical observations from uncut gems
Simple tests are used to help identify a gem.
hardness scale and other measurements. Although a somewhat destructive
test, hardness values
can be quite diagnostic.
preferential breakage along some surfaces e.g., mica. The planes
along which minerals break are planes where atomic bonds are weaker.
color of powdered material when scraped across a ceramic tile.
Note: Color and streak are not necessarily the same thing. For example,
has a black metallic luster, but streaks red.
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
|Mohs Hardness Scale
Observations from cut and uncut gems
Note that some of these measurements are only possible if the gem stone
is removed from its setting!
Generally unreliable! It MAY be a clue (note the luster).
Specific gravity is measured
by comparing the weight of the gem in air with its weight suspended in
density = amount of mass per given volume: measured as grams per cubic
centimeter. Specific gravity varies
with chemical composition and crystal structure type.
These measurements are relatively straight forward for stones
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.
(N is a known number)) and "CA" is the critical angle:
are the equipment used to measure the critical angle.
to measure the RI using a microscope.
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
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
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.
Measuring Specific Gravity
The critical angle
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
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
systems characterized by different shaped unit
Previous Lecture: What
is a Crystal?
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and Diamond Simulants