Optical Properties of Gemstones

Reflection, Refraction & Refractive Index

Light and gemstones

Light is a form of electromagnetic radiation. The visible spectrum — the portion our eyes detect — spans wavelengths from roughly 400 nm (violet) to 700 nm (red). In a vacuum, light travels at approximately 300 000 km/s. When it enters a denser medium such as a gemstone, it slows down and its behaviour changes in ways that are central to gemmological identification.

Reflection: light bouncing from a surface

Reflection happens when light strikes a surface and bounces back. In gemmology, surface reflection controls lustre — how bright the surface looks. The key rule is simple: angle of incidence = angle of reflection. This holds true regardless of the material or the wavelength of light.

Refraction: light bending as it enters a new material

Refraction happens when light passes between materials with different optical density, such as air to quartz or air to zircon. Light changes speed, so its path bends.

The normal is an imaginary line drawn at 90° to the surface. We measure: angle of incidence (incoming ray), angle of reflection (bounced ray), and angle of refraction (transmitted ray), all from this normal.

Refractive Index (RI)

Refractive index is the ratio of the speed of light in a vacuum to its speed in the material. A gem with RI 1.54 (quartz) slows light to roughly 65% of its vacuum speed; diamond at RI 2.42 slows it to about 41%. Higher RI generally means stronger bending, brighter lustre, and more brilliance.

RI is conventionally measured using sodium light at 589.3 nm (the Na D line). This standard wavelength allows consistent, comparable readings across different instruments and laboratories.

Interactive Snell Lab

Critical Angle & Total Internal Reflection

When light inside a gem reaches the surface at a steep enough angle, no transmitted ray escapes. Instead, light is reflected back inside the gem. This is total internal reflection (TIR).

The critical angle is the minimum angle of incidence (measured from the normal) at which TIR occurs. Its value depends on the RI contrast between the gem and the surrounding medium.

Dispersion, Fire, Reflectivity & Lustre

Dispersion means different wavelengths of light bend by different amounts when passing through a gem. Blue light generally bends more than red, splitting white light into a spectrum of colours. In faceted gems, this creates the spectral flashes known as fire.

Dispersion is measured as the difference in RI between two standard wavelengths. In gemmology, the conventional interval is from the Fraunhofer B-line (686.7 nm, red) to the G-line (430.8 nm, violet). This B–G value is the standard dispersion figure quoted for gems.

Reflectivity at a polished surface increases with RI. At normal incidence, the fraction of light reflected is determined by the Fresnel relationship. In practical terms, this means higher-RI stones have stronger surface lustre.

Double Refraction, Optic Axis & Optical Character

Double refraction (DR) is the phenomenon where a single incoming light path splits into two inside an anisotropic gem. The two rays travel at different speeds and in slightly different directions, emerging as separate beams.

Isotropic materials (cubic crystals and amorphous substances) are singly refractive — light has one RI in all directions. Anisotropic materials (all other crystal systems) are direction-dependent and produce two refracted rays.

Ordinary and extraordinary rays

In uniaxial crystals, the two rays are called the ordinary ray (ω) and the extraordinary ray (ε). The ordinary ray obeys Snell’s law in all directions and has a constant RI (n_ω). The extraordinary ray’s RI (n_ε) varies with direction, reaching its extreme value perpendicular to the optic axis.

Birefringence: the number, not the phenomenon

Birefringence is the numerical difference between two RI values in a doubly refractive stone. Double refraction is what you see; birefringence is the measured RI separation.

Optic axis / optic axes

The optic axis is a direction of single refraction in an otherwise doubly refractive crystal. Along that direction, both rays travel at the same speed and the splitting effect disappears.

Biaxial crystals

Biaxial crystals (orthorhombic, monoclinic, triclinic) have three principal RI values, conventionally labelled n_α, n_β, and n_γ (where α < β < γ). There are two optic axes — two directions along which both rays travel at the speed corresponding to n_β.

In practical gemmology, biaxial behaviour shows up as a more complex pattern on the refractometer: two shadow edges that both move as the stone is rotated, with the mid-range value (β) remaining roughly constant.

Optical character comparison

Optic Sign

Optic sign (positive or negative) indicates which ray has the higher RI:

• Uniaxial + (positive): n_ε > n_ω — the extraordinary ray is the slower ray. Examples: quartz, zircon.
• Uniaxial − (negative): n_ε < n_ω — the ordinary ray is the slower ray. Examples: corundum, tourmaline, calcite.
• Biaxial +: n_β is closer to n_α.
• Biaxial −: n_β is closer to n_γ.

Optic sign is a more advanced distinction, but it can separate gems that otherwise share similar RI and birefringence values.

Polarisation & Pleochroism

Polarisation of light

Normal white light vibrates in all directions perpendicular to its path. When it passes through a doubly refracting gem, the two emerging rays are plane-polarised — each vibrates in one direction only, and the two vibration planes are at right angles to each other.

A polarising filter (Polaroid) transmits only one vibration direction. By using two crossed polarising filters, we can test whether a gem is singly or doubly refractive.

The polariscope

A polariscope consists of two polarising plates: the polariser (lower) and the analyser (upper), set with their vibration directions crossed at 90°. When no stone is present, the field goes dark because the second filter blocks the light transmitted by the first.

• Isotropic gems remain dark in all rotation positions — they do not change the polarisation state and are singly refractive.
• Anisotropic gems show alternating light and dark positions as the stone is rotated — four extinction positions per 360° rotation, roughly 90° apart.
• ADR caution: garnet and spinel (normally isotropic) may show patchy bright areas due to anomalous double refraction from internal strain — do not confuse this with true anisotropy.

Pleochroism

Because the two polarised rays in a doubly refracting gem can be absorbed differently, they may carry different colour intensities. This directional colour change is pleochroism.

Uniaxial stones can show two colours (dichroism). Biaxial stones can potentially show three colours (trichroism), one for each vibration direction (α, β, γ).

The dichroscope

A calcite dichroscope uses a rhomb of Iceland spar (calcite) to separate the two polarised rays side by side in two adjacent windows. When viewing a pleochroic gem, the windows show different colours or intensities.

Refractometer Use in Practical Gemmology

How the refractometer works

The standard gemmological refractometer uses a high-RI glass prism (typically lead glass, RI ≈ 1.79–1.81) shaped as a hemicylinder. Light enters the prism at varying angles. When it reaches the prism–gem boundary below the critical angle, it passes into the gem and is lost. When it reaches the boundary above the critical angle, total internal reflection sends it back. The boundary between light and dark on the viewing scale — the shadow edge — corresponds directly to the gem’s RI.

Contact liquid

Contact liquid is essential. A thin film (typically methylene iodide, RI ≈ 1.79, or a synthetic equivalent) bridges the air gap between the flat gem facet and the curved prism surface. Without it, the shadow edge is weak or invisible.

Monochromatic vs. white light

Sodium light (589 nm) produces a single sharp, crisp shadow edge — ideal for precise RI readings. White light produces a broader edge with coloured fringes because each wavelength has a slightly different RI. When white light is used, the reading is taken at the green–yellow boundary of the fringe, which approximates the sodium-light RI.

Basic reading logic

Step-by-step workflow

1. Clean prism and facet thoroughly.
2. Apply a tiny drop of contact liquid.
3. Place gem facet-down on the prism.
4. Illuminate with correct yellow/monochromatic light.
5. Focus and read the shadow edge position.
6. Rotate the stone and observe edge movement.
7. Record low/high RI and birefringence where relevant.

Absorption Spectra & the Spectroscope

Selective absorption

When white light passes through a coloured gemstone, certain wavelengths are selectively absorbed by the chemical elements present — particularly transition metals such as chromium, iron, vanadium, and manganese. The remaining wavelengths that reach your eye determine the stone’s perceived colour.

A spectroscope spreads transmitted light into a spectrum, revealing this absorption as dark bands or lines against a continuous rainbow background. Each gem species and colour variety may show a distinctive pattern that aids identification.

Types of hand spectroscope

• Prism spectroscope: uses a glass prism to spread the spectrum. Gives better resolution in the red end of the spectrum, which is important for chromium-related absorption. The scale is uneven — the blue end is spread wider.
• Diffraction-grating spectroscope: uses a finely ruled grating. Gives an evenly spaced scale across the spectrum, making wavelength estimation easier, but generally lower resolution than a prism instrument.

Key diagnostic spectra

Practical tips

1. Use a strong, focused light source (fibre-optic or LED pen light) positioned behind or beside the stone.
2. First observe the spectrum with no stone to learn the normal continuous colour spread.
3. Place the stone in the light path and look for dark bands or lines against the rainbow background.
4. Narrow the spectroscope slit for finer detail; widen it for more light when absorption is faint.
5. Record approximate wavelength positions of any bands — even rough notes help narrow identification.

How Reflection and Refraction Help Identify Gems

In practical testing, RI and refraction behaviour are combined with other evidence, not used alone.

• Separate look-alike stones quickly by RI range.
• Distinguish singly refractive vs doubly refractive behaviour.
• Estimate optical character (isotropic, uniaxial, biaxial).
• Support conclusions with SG, polariscope, microscope, spectroscopy, and inclusions.