Gem Info

Gems are structured materials. Their colour, brilliance, durability and optical behaviour arise from the interaction of chemical composition, atomic arrangement, crystal symmetry, crystal defects, growth history and cutting orientation.

Whether a gem formed naturally within the Earth or was grown under controlled laboratory conditions, these same crystallographic principles govern its physical and optical behaviour.

Two gems may appear similar while differing substantially in mineral species, refractive index, hardness or internal structure. Conversely, very different colours may belong to the same mineral, produced by only minor chemical substitutions or lattice defects.

This page introduces the mineralogical principles behind the gems featured by Gemdrop®: sapphire, ruby, Space Diamond moissanite, emerald, opal, alexandrite and cubic zirconia.

From atoms to gems

A crystalline gem consists of atoms or ions arranged in an ordered, repeating three-dimensional lattice. The smallest repeating structural unit is the unit cell.

That internal architecture controls many of the properties measured by gemmologists:

• refractive index and birefringence;
• hardness, cleavage and density;
• crystal habit and growth zoning;
• pleochroism and fluorescence;
• thermal and electrical behaviour;
• and the incorporation of colour-producing elements.

A gem’s physical properties are therefore the macroscopic expression of its chemistry and structure. Chemical formula alone is not enough: the arrangement and bonding of the atoms are equally important.

One mineral, many colours

Ruby and sapphire provide the clearest example.

Both are corundum, crystalline aluminium oxide, $Al_2O_3$. Their basic structure is the same, but small concentrations of trace elements alter the absorption of visible light.

• Ruby is red corundum, coloured principally by chromium.
• Blue sapphire commonly derives colour from interactions involving iron and titanium.
• Other sapphire colours may involve chromium, iron, vanadium (corundum-alexandrite) or related defects and charge-transfer processes.

Emerald is the green variety of beryl, with colour generally associated with chromium and/or vanadium occupying structural sites.

Alexandrite is a chromium-bearing variety of chrysoberyl. Its colour-change effect results from selective absorption combined with differences between daylight-rich and incandescent-rich illumination.

Colour may also arise from lattice defects, trapped electrons or deliberately introduced dopants. This is especially relevant to coloured cubic zirconia and some varieties of synthetic moissanite.

Crystal systems and optical character

Crystal symmetry determines whether a gem is optically isotropic or anisotropic.

Ruby and sapphire

Corundum belongs to the trigonal crystal system. It is optically uniaxial, meaning that it has one optic axis and two principal refractive indices.

This produces measurable birefringence and may create pleochroism, particularly in strongly coloured material.

Emerald

Beryl belongs to the hexagonal system and is also optically uniaxial. Emerald commonly displays directional colour differences related to crystallographic orientation.

Space Diamond [Moissanite]

Moissanite is crystalline silicon carbide, $SiC$. Gem-quality synthetic moissanite is commonly based on hexagonal or closely related polytypic structures. It is anisotropic and strongly birefringent.

Its high refractive index and exceptionally strong dispersion generate intense brilliance and spectral fire. Its birefringence may also produce visible doubling of internal facet junctions in some viewing directions.

Alexandrite

Chrysoberyl is orthorhombic and optically biaxial. Biaxial crystals possess three principal refractive indices:

$$n_\alpha < n_\beta < n_\gamma$$

These correspond to the mutually perpendicular optical vibration directions $X$, $Y$ and $Z$. The plane containing the two optic axes is known as the optic axial plane.

This directional optical structure contributes to alexandrite’s pleochroism and affects the colour presented by a cut gem.

Cubic zirconia

Cubic zirconia is stabilised crystalline zirconium oxide, generally $ZrO_2$ with added stabilising oxides. Its cubic symmetry makes it optically isotropic under ideal conditions.

It has high dispersion, strong brilliance and a substantially greater density than diamond or moissanite.

Opal

Opal differs fundamentally from the crystalline gems above. It is hydrated silica with variable structural order and is commonly classified as a mineraloid rather than a conventional crystal.

Precious opal’s play-of-colour is produced by diffraction from ordered arrays of microscopic silica spheres. Its colour is therefore structural rather than arising solely from trace-element absorption.

Crystal orientation survives cutting

Faceting changes the external shape of a gem, but it does not alter the orientation of its internal lattice.

Crystallographic planes may be represented using Miller indices, such as $(100)$, $(010)$, $(101)$ or $(210)$. These describe the orientation of planes relative to crystallographic axes—not their physical size.

Orientation matters because it influences:

• pleochroic face-up colour;
• extinction and light return;
• visible birefringent doubling;
• cleavage risk;
• growth zoning;
• and cutting yield.

This is particularly important for sapphire, ruby, emerald and alexandrite. A carefully chosen orientation can improve colour and optical performance, while an unsuitable orientation may produce uneven colour, excessive darkness or reduced brilliance.

How lab-grown gems form

Lab-grown gems are not simply visual imitations. Where a synthetic counterpart exists, it has substantially the same chemical composition and crystal structure as the corresponding natural mineral.

Relevant growth technologies include:

• Flame fusion: widely used for lab-grown ruby and sapphire.
• Czochralski pulling: used for high-quality corundum and alexandrite-type crystals.
• Flux growth: capable of producing emerald and alexandrite with complex internal growth features.
• Hydrothermal growth: particularly important for synthetic emerald.
• High-temperature crystal growth: used in the production of moissanite.
• Controlled melt crystallisation: used for cubic zirconia.
• Structured silica synthesis: used to produce synthetic opal with ordered sphere arrays.

Growth method may influence zoning, inclusions, strain, fluorescence and microscopic features, but the material must first be identified by its fundamental structure and properties.

Inclusions, defects and optical effects

Perfect crystals are theoretical ideals. Real gems contain substitutions, vacancies, growth boundaries, strain, inclusions and other departures from perfect periodicity.

These features may influence:

• colour;
• clarity;
• fluorescence;
• toughness;
• spectral absorption;
• and identification of growth history.

Inclusions are not merely flaws. They may preserve evidence of crystallisation, laboratory growth, treatment or later alteration.

In opal, microscopic order generates play-of-colour. In alexandrite, chromium substitution produces selective absorption. In corundum, trace elements and structural defects create the spectrum of ruby and sapphire colours. In moissanite and cubic zirconia, controlled impurities and growth conditions can produce precise colours unavailable or uncommon in nature.

How gems are identified

Scientific gem identification combines multiple independent observations.

Routine methods may include:

• optical microscopy;
• refractive-index measurement;
• polariscope examination;
• pleochroism testing;
• ultraviolet fluorescence;
• spectroscopy;
• specific gravity;
• and thermal or electrical conductivity.

Advanced questions may require Raman, FTIR, UV-Vis, photoluminescence, X-ray diffraction or compositional analysis.

No single observation should be treated as conclusive. A responsible assessment separates:

1. what was directly observed;
2. what was quantitatively measured;
3. what the combined evidence supports;
4. and what requires specialist laboratory confirmation.

Explore our gem guides

Continue to the individual guides for:

Sapphire
Ruby
Space Diamond [Moissanite]
Emerald
Opal
Alexandrite
Cubic Zirconia

Each guide examines the material’s composition, crystal structure, optical properties, growth method, durability, identification and care.

A polished gem may conceal its original crystal form, but it cannot escape its crystallography. Its internal structure remains the hidden architecture governing how it interacts with light, responds to wear and reveals its identity.