source adapted to html based on notes of Stephen A. Nelson, Tulane
includes information written and developed by Charles Lewton-Brain,
of the Ganoksin Project for Jewellers and author. His web site is http://brainpress.com/index.html
The optical characteristics and properties of gemstones often provide the
fastest and best methods of identification. A certain amount of theory is
necessary as optical principles determine cutting methods, gemstone
attributes and the function of gem testing instruments.
Light and our perception of it play a crucial role in our appreciation of
and identification of gemstones. Visible light however comprises only a
small part of what is referred to as the electromagnetic spectrum.
While the wave or undulatory theory of light has been mostly superseded by
the quantum (particle) theory the wave theory best serves the purpose of
describing light for gemmology. We can consider the electromagnetic
spectrum to consist of an infinite number of types of wavelengths, from
short to very long. Different wavelengths have different powers of
penetration dependent upon their length relative to the medium they pass
through. X-rays for example with a wavelength near atomic sizes pass
through or between most atoms. The amount passed depends upon the mass of
the atom concerned. Dense atoms like lead for instance provide a screen
against x-rays. An application of this is a test for diamonds, whether set
or unset, where the suspect stones are x-rayed for ten seconds over
photographic paper. Carbon atoms are small (low mass) and so diamond is
transparent to x-rays and is invisible on the photograph while all diamond
simulants show up as positive, opaque shapes.
A rough wavelength scale follows:
Note what a small portion of the spectrum comprises visible light. Light
can be thought of as progressing outward in a single path (a ray). The ray
forms a wave vibrating in all planes at right angles to the direction of
travel, the line of the ray.
White light is composed of a mixture of a great many wavelengths each of
which is perceived as a different colour. The wavelength of violet light
for example is about half that of red light.
The wavelengths of white light may be divided into:
Red 700.0 nm to 640.0 nm
Orange 640.0 nm to 595.0 nm
Yellow 595.0 nm to 575.0 nm
Green 575.0 nm to 500.0 nm
Blue 500.0 nm to 440.0 nm
Violet 440.0 nm to 400.0 nm
Refers to the ease with which light is transmitted through a substance.
Classifications of transparency in cut gemstones include: 1. Transparent
stones. An object viewed through the gem shows outlines clearly and
distinctly (diamond, topaz, corundum).
2. Semi-transparent. Blurred outlines of object but a great deal of light
still passes through the stone, i.e. chalcedony.
3. Translucent. Some light passes through, no object can be seen through
stone, i.e. opal, some jades, much cryptocrystalline quartz.
4. Semi-translucent. Light is only transmitted through edges, where they
are thin, i.e. turquoise.
5. Opaque. No light passes through, i.e. malachite, pyrites.
Colour and degree of colour will affect transparency as will inclusions,
flaws, etc. Quality will also affect it. The characteristics are
subjective in nature and overlap exists.
Reflection of Light
If a ray of light falls onto a plane mirror the light is reflected away
from the surface. The angle of incidence NOI equals the angle of
reflection NOR and IO, NO and RO are in the same plane. All angles in
optics are measured from the ‘normal’, an imaginary line at right angles
to the surface at the point of incidence (where the light ray strikes the
A ray light entering an optically denser medium is bent (refracted)
towards the normal. The greater the bending (refraction) for a given angle
of incidence the greater is the refractive power of the stone.
The cause of refraction is that the light waves (300,000 km/second) are
slowed down as they enter the optically denser medium. In the 17th century
Snell (Dutch scientist) described laws relating angles of incidence and
refraction for two media. There is a constant ratio between the sines of
these angles for any given two media. The constant ratio obtained is
called the refractive index. Air is chosen as the rarer medium and yellow
sodium light is the standard for refractive index measurements. Refractive
index is a measure of a gem’s refractive power. It is the ratio of the
sine of the angle of incidence divided by the sine of the angle of
refraction when light passes from air into the denser medium.
Gems refractive indices range from under 1.5 to over 2.8.
Total Internal Reflection
A ray passing in the opposite direction, from the denser to the rarer (gem
to air) medium is bent (refracted) away from the normal.
As the angle of incidence is increased the angle of refraction away from
the normal increases until a point is reached when the ray I1OR1 exits
parallel to the table of the stone. Any further increase in this angle
causes the ray to be totally reflected back into the gem. Ray I2OR2 has
been reflected back into the gemstone. This is called total internal
reflection and the angle I1OM is called the critical angle for the medium
in question. The brilliant cut of diamonds uses total internal reflection
and the critical angle for diamond and air to ensure that all light
entering the stone is totally reflected and passes out the table or crown
facets of the stone. The critical angle is also what enables a
refractometer to differentiate gemstones of different species.
A white light ray entering an optically denser medium and leaving by a
plane inclined to that of entry will have its colours separated, analyzed,
spread out. This is because each colour has a different wavelength and so
is differently slowed down (refracted) by the medium. Red (longest
wavelength) is slowed the least and violet (shortest wavelength) the most.
This spreading is termed dispersion. In gemstones the effect gives rise to
the stone’s ‘fire’. It may be measured with complex equipment and
numerical values given. The higher the number the greater the fire where
the stone’s colour does not mask the effect, as in demantoid (green)
garnet with a greater dispersion (.057) than diamond (.044). With practice
and standard stones numerical estimates of dispersion may be made with the
Hanneman/Hodgkinson slit technique.
Plane Polarized Light
When a light ray passes through a doubly refractive gemstone it is split
into two rays with different amounts of refraction. Each ray is plane
polarized, that is instead of the wave vibrating all directions about the
line of the ray it vibrates in a single plane only. Each ray is plane
polarized at right angles to the other. As each ray is differently
refracted so it is differently absorbed by the stone and possesses in
coloured gems a different hue or colour.
The Dichroscope picks up each ray at the same time and allows one to view
them side by side. A simple dichroscope is a block of calcite with black
paper glued to one end which has a small rectangular hole cut in it. The
viewer sees two images because the light ray has been split by the high
double refraction of calcite. Each image is of a different ray (each ray
is also plane polarized at right angles to the other - this is what allows
the calcite to present them separately). If a difference in colour exists
it will be visible by comparison. One must always test in several
directions. This can be of some use in identifying gemstones by their
characteristic dichroic or trichroic colours but is usually used as a
method of detecting double refraction. Presence of dichroism proves double
refraction. Absence does not mean a material is not doubly refractive - it
may be that the dichroism is very weak, or in transparent stones there is
none evident. It can be used to find an optic axis. If three colours
(trichroic) are seen it means the stone is biaxial. If two only are seen
it is uniaxial. Transmitted, not reflected light must be used as reflected
light may be partly polarized. Most natural corundum is cut with the table
oriented to the optic axis and will show no dichroism through the table.
Most synthetic corundum has the table parallel to the optic axis and
dichroism is strongest through the table. This is then an indication of
A mineral may be defined as a homogenous substance produced by the
processes of inorganic nature having a chemical composition and physical
properties which are constant within narrow limits. Its structure is
crystalline. It is composed of the same substance throughout. Except for
impurities it has the same chemical formula for all specimens of the
mineral. Its atoms usually have a definite and ordered crystal structure.
What makes a mineral (or an organic product) a gemstone is cultural and
partly subjective: beauty, durability and rarity.
Minerals often occur in geometrical forms bounded by plane surfaces. These
are crystals and the internal structure determines properties which allow
the identification of the gem material; its differentiation from other
minerals, imitations and sometimes synthetics. Crystals have:
An orderly and symmetrical atomic structure.
A definite external geometrical shape bounded by plane faces.
Physical (and optical) properties which vary with direction.
No regular atomic structure.
No tendency to assume a definite external shape.
Properties which are the same in all directions.
(the above derived from the British Gemmological Association) Crystalline Material:
Possesses the regular structure and directional properties of a crystal
but not the regular geometrical shape. Also called massive. e.g. rose
quartz. Crypto-crystalline Material:
Material which consists of a multitude of tiny, often submicroscopic
crystals. e.g. Chalcedony.
Crystal symmetry refers to the balanced pattern of the atomic structure
which is reflected in the external (crystal) shape.
Different species vary in the symmetrical arrangement of faces. These
arrangements have certain 'planes' and 'axes' of symmetry.
These form part of the definition of the crystal system to which specific
gemstones belong. Plane of symmetry:
An imaginary plane dividing a body into two parts such that each is the
reflected image of the other. Crystals may have more than one plane of
symmetry. i.e. a cube has nine planes of symmetry.
Axis of symmetry:
An imaginary axis is placed through a perfect crystal so that during a
single rotation about this axis the outline of the crystal form appears
identically more than once; 2, 3, 4 or 6 times.
Centre of symmetry: (centro-symmetry)
Often present, it exists when every face of a perfect crystal is exactly
opposite a similar face on the other side of the crystal.
An n-fold (Cn) proper rotation operation
represents a counter-clockwise movement of (360/n)° around an axis through
the object. If an n-fold rotation operation is repeated n
times, then the object returns to its original position. Crystals with a
periodic lattice can only have axes with 1-, 2-, 3-, 4-, and 6-fold
symmetry axes. The following description of rotational symmetry operations
is similar to that given by Prof. Stephen Nelson.3 In the
following drawings, the symmetry axis extends perpendicular from the page.
1-Fold Rotation. A 1(E)-fold rotation operation implies
either a 0° rotation or a 360° rotation, and is referred to as the identity
2-Fold Rotation. A 2-fold(C2) rotation operation moves
the object by (360/2) ° = 180 °. The symbol used to designate a 2-fold
axis is a solid oval.
Crystal axes: (Crystallographic axes)
To describe crystals imaginary lines are used intersecting at 0 (the
origin). These are specific to the various crystal systems, intersecting
at given angles and being of given lengths specific to each crystal
The intersection of the crystal axes. Habit:
Gemstone species tend to occur in characteristic shapes which relate to
one or more of the forms common to the crystal system of the gemstone in
question. The crystal form or forms which a gemstone most often appear are
it's habit. e.g. diamond: octahedron, emerald: 6 sided prism. Form:
Those faces of a crystal which are identically related to the crystal
axes. When the space so defined is completely enclosed (cube, octahedron)
it is a closed form. When identical faces do not completely enclose the
space (four or six sided prism; top and bottom open) it is an open form. Twinned crystals: (compound crystals)
A twin is a single crystal composed of two or more parts with any part in
reversed structural orientation to the next, or interpenetrated. Contact twin:
Sharing a common plane.
Two individuals have grown from a common origin and appear to penetrate
each other. e.g. cross stones.
A series of contact twins often as extremely thin plates. Atoms in
adjacent sheets are reversed, i.e. alternate plates are in the same order.
This can give rise to special optical effects as in the feldspar
labradorite. Secondary twinning or parting:
The crystal is composed of very thin plates parallel to definite
crystallographic directions. e.g. ruby, this gives rise to 'false
Three crystal axes of equal length intersect at right angles to each
other. e.g. diamond, spinel, garnets. Tetragonal Three axes intersect at
right angles to each other. The vertical axis is of unequal length while
the two horizontal axes are of equal length. e.g. zircon, rutile. Hexagonal
Four crystal axes. Three are of equal length and intersect at 60o to form
a horizontal plane which the fourth intersects at right angles. The
vertical fourth is of unequal length and forms an axis of 6-fold symmetry.
e.g. Beryl, apatite. Trigonal
Four crystal axes. Three of equal length intersecting to form a horizontal
plane which is intersected at right angles by the fourth axis. The
vertical fourth is of unequal length and forms an axis of 3-fold symmetry.
e.g. quartz, corundum, tourmaline, dioptase, haematite. Orthorhombic (Rhombic)
Three crystal axes of unequal length interest each other at right angles.
e.g. topaz, peridot, Chysoberyl, iolite, sinhalite, andalusite. Monoclinic
Three axes. Two of unequal length intersect each other obliquely to form a
plane which is intersected by the vertical third (of unequal length) at
right angles. e.g. jadeite, nephrite, diopside, orthoclase feldspar,
serpentine, sphene, malachite, spodumene. Triclinic
Three axes of unequal length intersect each other at oblique angles. e.g.
NOTE: The Gemological Institute of America (GIA)
system for crystal types differs from the above. Contact their web site
for information on their courses, books and so on.
some of the terms used in discussing optical effects in gemstones.
Lustre (or the American spelling Luster) refers to the amount and quality
of light reflecting from a gem’s surface to the eye. It is partially a
subjective measurement but can be helped by comparison with a standard set
of gems with known lustres. The hardness of a material, its refractive
index and the degree to which it has been polished will have a bearing on
the lustre. In general the harder a material is the higher the lustre, the
softer it is the lower the lustre. The Americans and the British use
slightly different nomenclature for lustres. The American Liddicoat terms
the categories: “metallic, submetallic, adamantine, subadamantine,
vitreous, subvitreous, waxy, greasy, silky, dull.” He goes on to say:
‘“The first three reflect the presence of refractive indices over the
refractometer scale. Subadamantine suggests an index high on the scale;
vitreous, midscale; and subvitreous, low. Waxy and greasy lusters are
usually associated with poorly polished surfaces, while silky refers to
stones with many needle like inclusions.” (Liddicoat, ‘Handbook of Gem
Identification’, pp 216, 1993 ed.)
Britain’s Webster says that many gems have a glassy or vitreous lustre. He
gives examples and lists the lustre types as: “Metallic: silver,
Adamantine: diamond, Subadamantine: demantoid garnet, Resinous adamantine:
certain zircons, Vitreous: quartz, Resinous: amber, Silky: fibrous
materials such as satin-spar, Pearly: usually seen only on cleavage faces,
John Sinkakas makes a correlation between refractive index and luster
Refractive index is given first, then the corresponding comment on luster.
1.3-1.4 Poor reflections, inclined to be greasy or oily in appearance
1.5-1./8 Brightly reflective, like glass
1.6-1.9 Resinous in appearance
1.9-2.5 Very brightly reflective, adamantine, sometimes appearing as if
the mineral is lightly coated with a metal film.
2.5 + Submetallic, bright luster, definitely metallic in appearance
(Sinkakas, John,. “Gemstone and Mineral Data Book”, pp 336)
Sheen is due to the reflection of light from material below the surface of
a gem. Moonstone, spectrolite and other feldspars are examples. Sheen in
moonstone is also called schiller or adularescence. Pearls too have sheen
as light reflects from below the surface of the pearl.
Interference of Light
When a light ray strikes a surface composed of thin films part is
reflected and part refracted into the films. The ray then reflects from
film levels below the top surface and reenters the air. As it does so it
interferes with; either intensifying or quenching certain wavelengths
(colors) in other light rays reflecting from the top of the film. This
produces color and light effects like that of oil on water, soap bubbles,
Titanium and Niobium coloring, labradorite, tempering colors on steel and
so on. In the diagram below a single ray is shown but in reality an
infinite number of rays are doing the same thing simultaneously at all
points on the surface of the partially reflective top layer or film.
A general term for color effects produced by interference or by
diffraction. Color play in opals, mother of pearl etc. are examples.
Play of Colour
A term used to describe the colors seen in opal. This is caused by light
diffraction from a regular structure of silica spheres in opals.
When light passes over many tiny sharp edges or between many repeated
points of differently refracting media an interference like phenomenon
occurs; light is spread out into specific colors. You can see this on
music CDs and sometimes on mesh between you and a light source. This
principle is used in the diffraction grating spectroscope. This is what
causes the play of color in opals, light being bent and diffracted as it
passes innumerable regular stacked layers of minuscule silica gel spheres.
The milkiness of opals. Sometimes it is used to describe play of color.
When a gem material contains many minute fibrous inclusions oriented in
one direction and it is cut en cabachon a streak of light or ‘eye’ can be
seen at right angles to the direction of the inclusions. An example used
to explain this is the light streak visible on a spool of silk thread or
on an old 35 RPM record. Examples include chrysoberyl (cymophane),
crocidolite (tigers eye) and quartz. Many gems can exhibit an ‘eye’
including tourmaline, beryl, nephrite, jadeite etc.
Star stones, these are most commonly sapphire and ruby but may include
garnet, spinel, diopside and other gems. It is a special type of
chatoyancy as the cause is due to many small fibrous inclusions oriented
at set angles to each other. Examples are ruby (60o), garnet (70o). These
inclusions in the case of corundum are all parallel to the lateral axes of
the crystal and at right angles to the vertical crystal axis. When the
stone is cut with the top of the cabachon dome oriented with the main
crystal axis passing vertically through it and the silk inclusions
parallel to the girdle of the stone asterism results. Each set of silk has
a streak of light at right angles to it and a star is seen.
Along with the microscope and refractometer this is a major identification
tool in gemology. As light passes through a gem the presence of certain
chemicals will cause specific wavelengths of light to be absorbed.
Instances also occur where wavelengths are intensified or the stone
actually emits light (fluorescence wavelengths - rubies, spinel). When
light is spread out by a prism or diffraction grating spectroscope into a
wide band these absorbed wavelengths show up a lines or areas of darkness
in the spectrum. While the actual wavelength numbers can be used in
identification usually only a pattern of lines is used to identify the
stone. It can be the fastest way of checking out large numbers of stones,
even small ones, especially red gems, as spinel, ruby and tourmaline have
distinctive spectra. It can be used to identify synthetic verneuil
sapphire, blue synthetic spinel, almandine garnet to name a few. Note that
British gemmologists have the red on the left and Americans have it on the
right when looking at spectra. Here is an example of what the absorbtion
spectrum pattern of a gem stone might look like through a spectroscope.
Red is on the right in the diagram below.
‘Gem Testing’ by Anderson and Webster,
‘Handbook of Gem Identification’ by Richard Liddicoat.
The latter has the best group of drawings available on what spectra look
like, very fuzzy, hard to see at times.
Anderson, B.W. Gem Testing. London: Butterworths, 1980.
Liddicoat, Richard T., Jr. “Handbook of Gem Identification”. 12th ed.
Santa Monica: Gemological Institute of America, 1993.
Sinkakas, John,. “Gemstone and Mineral Data Book: A Compilation of Data,
Recipes, Formulas and Instructions for the Mineralogist, Gemologist,
Lapidary, Jeweler, Craftsman and Collector”. Prescott, AZ: Geosciences
Webster, Robert. “Gems: Their Sources, Descriptions and Identification”.
Fourth ed. Rev. B.W. Anderson. London: Butterworths, 1983.