Optical Properties of Gemstone

In the identification of gemstones, a proper study of their optical property has always been found to be of great value. Accurate measurement of the optical properties of gemstone is very useful since they are extremely sensitive to minute changes in composition and strain in the crystal structure. Precise identification can be made conveniently with reference to their optical properties.

Certain characters of minerals depend upon light. For a detailed accurate study and proper understanding of optical characters of the mineral, knowledge of the following terminology is very essential.

  1. Light: It travels in the form of waves, like ripples in a pond. It has a dual nature i.e., it behaves as a wave, as well as a stream of particles (energy). In the production of light, energy is derived from some other form of energy. Visible light, whether emitted by the sun or a candle, is a form of radiant energy. The portion of the sun's radiant energy spectrum that we can see is only a very small part of the Electromagnetic Spectrum, the remainder being invisible radiations.
    • The electromagnetic spectrum includes the high frequency (short wavelength) cosmic rays, gamma rays, X-rays, ultra-violet rays, visible light, infrared rays, radar and television waves, the radio and electrical waves (long wavelength).
    • All these forms of radiation travel in air at about the same speed (186,300 miles per sec.) and travel in a wave form. The frequency and the wavelength determine the type of radiation i.e. the region of the electro-magnetic spectrum. The longer the wavelength, the lower is the frequency and the shorter the wavelength, the higher is the frequency.
    • The units of measurement used for wavelength (also used to measure very small stances like atomic bonding) are the Angstrom Unit and the Nanometer.
      • 1 Angstrom Unit (A.U.) = one ten millionth of a millimetre (10−7mm).
      • 1 Nanometer = 10−6 mm = 10 Angstrom units, (e.g. the mean wavelength of sodium light is expressed as 5893 A.U. or 589.3 nm)
    • The following table gives approximate wavelengths, frequencies and energies for certain regions of the electromagnetic spectrum.
     
    Electromagnetic Spectrum
    Region Wavelenght (Angstroms) Frequency (Hz) Energy (eV)
    Radio > 109 < 3 × 109 < 10−5
    Infrared 106 - 7000 3 × 1012 - 4.3 × 1014 0.01 - 2
    Visible 7000 - 4000 4.3 × 1014 - 7.5 × 1014 2 - 3
    Ultraviolet 4000 - 10 7.5 × 1014 - 3 × 1017 3 - 103
    X-Rays 10 - 0.1 3 × 1017 - 3 × 1019 103 - 105
    Gamma Rays < 0.1 > 3 × 1019 > 105
  2. Ordinary Light: In gemmology, the most important portion of the electromagnetic spectrum is that of visible light. The visible spectrum ranges from approximately 400 nm (4000 A.U.) to 700nm (7000 A.U.) and includes the seven primary colours - violet, indigo, blue, green, yellow, orange and red (VIBGYOR). When these wavelengths are mixed together we see them as white light.
  3. Polarised Light: Light obtained from any source (sun, lamp etc.) vibrates in all planes at right angles to the direction of propagation. But when this light is passed through a polaroid sheet, the vibrations get confined to a single plane. The light that emerges through the polaroid sheet is said to be polarised.
  4. Absorption: The colour of a stone is governed by absorption of some of the visible wavelengths. Absorption occurs due to the atomic arrangements in particular substances. When any radiation is incident on a substance, some wavelengths are absorbed by the sub-units (electrons) of atoms while the unabsorbed wavelengths are transmitted, resulting in a particular colour being seen. Some mineral absorb different wavelengths from light vibrating in different planes. Such minerals are said to be pleochroic and the property is called pleochroism.
  5. Reflection: When a ray of light that strikes a polished surface, is bounced back into the same medium, it is said to be reflected.
  6. Refraction: When light passes obliquely from one medium to another, in which it .ravels with a 'different velocity', it undergoes an abrupt change in direction. The change in the direction of light that occurs at the contact surface between two media, such as air and water, is known as Refraction.
  7. Isotropic: When any refracted ray is transmitted as a single ray in all directions within a substance, the substance is said to be Isotropic.
  8. Anisotropic: When an incident ray is refracted as two rays within a substance such that the vibration direction of one of the rays is perpendicular to that of the other, it is called an anisotropic substance.
  9. Optic Axis: In an anisotropic stone, the refracted ray may be transmitted as a single ray i.e. without splitting, in one or two particular directions. Such a direction of single refraction in a doubly refracting substance is called an optic axis direction, i.e. a direction along which an anisotropic substance behaves as if it were isotropic, is called the optic axis.

The qualities that determine the beauty of a gemstone are directly related to the appearance of the stone when light is transmitted through the stone or when it is returned from the surface.

Reflection of Light

It can be best observed on a smooth surface. It is defined by the terms - incident ray (I), normal (N) and reflected ray (R). When a ray falls on a surface the falling ray is called the incident ray while the ray which goes back after striking the surface, is the reflected ray. An imaginary line perpendicular to the surface at the point of contact of the rays with the surface is called the normal.

There are two laws of reflection which state that:

  • The incident-ray, the normal and the reflected ray lie in the same plane.
  • The angle of incidence is equal to the angle of reflection.

The phenomenon of reflection of light is responsible for optical effects such as lustre, sheen, chatoyancy, schiller, asterism, aventurescence etc.

  1. Lustre: The quantity and quality of light reflected off the surface of the stone, give rise to specific lustre such as metallic, adamantine, sub-adamantine, vitreous, sub-vitreous, greasy, resinous, waxy, silky and pearly.
  2. Sheen: Reflection of light from inclusions or layers below the surface of the stone moving white light effect as schiller in moonstone, milky white sheen from silk inclusions in ruby and sapphire (referred to as milky-ness in corundum etc.)
  3. Aventurescence: Reflection of light from metallic platelets or inclusions, resulting in a shimmering, glittery effect as in the case of aventurine quartz, sunstone feldspar, goldstone glass etc.
  4. Schiller: Similar in effect to aventurescence, it is the technical name given to stones that, by virtue of many flat separations or partings, usually along cleavage planes, show a strong sheen - like reflection in certain positions.
  5. Chatoyancy (Cat's Eye): Reflection of light from one set of parallel inclusions (needles or crystals) or structural planes (twin planes and / or cleavage planes), which results
    • in a single moving white line on the surface of a stone,
    • when the stone is cut in a cabochon or domed cut,
    • the eye is seen at right angles to the direction of the inclusions. This is commonly seen in chrysoberyl cat's eye, apatite cat's eye, quartz cat's eye.
  6. Asterrism (Star Effect): This is merely an extension of the cat's eye effect, in that, the presence of tow sets or three sets of parallel inclusions result in 4-rayed or 6-rayed stars when the stone is cut as a cabochon.
    • Asterism can be observed in reflected light and with transmitted light. When seen in reflected light as in the case of the star ruby, star diopside etc. it is know as Epiasterism. Star rose quartz and some pyrope - almandine star garnet, exhibit asterism in both reflected and transmitted light. Such stones are said to show Diasterism.

Refraction of Light

When a ray of light passes from an optically rarer medium ( air ), to an optically denser medium (gemstone), it bends towards or nearer to the normal and vice versa. In other words, the velocity of light changes with me optical density of the medium.

  • Light moves relatively slowly through an optically denser medium and hence bends towards the normal.
  • It travels relatively faster in an optically rarer medium and hence moves away from the normal.
  • When a ray of light strikes the surface at right angles i.e. at normal incidence there is no deviation and the light is transmitted through.

The Laws of refraction of light were laid down in the 17th century by the physicist Snell, and are known as Snell's Laws which are:

  1. The incident ray, the refracted ray and the normal at the point of incidence, lie in the same plane.
  2. The sine of the angle of incidence and the sine of the angle of refraction bear a definite ratio to each other, which depends upon
    • the optical densities of the two media in contact with each other
    • the wavelength of light being used.
      (sine i ÷ sine r) = R.I. = a constant (for a given set of media and particular wavelenght of light.)

When the light is incident from air, this constant is the Refractive index, of the denser medium (stone).