Ruby

Introduction

Ruby is one of the world's most treasured gemstones, admired for its rich red colour and remarkable durability. Although many people think ruby and sapphire are completely different gems, they are actually two varieties of the same mineral: corundum.

Pure corundum is colourless and is made from just two elements—aluminium and oxygen (Al₂O₃). Ruby forms when a very small number of aluminium atoms inside the crystal are replaced by chromium atoms. This tiny chemical change completely alters the way the crystal interacts with light, producing the vivid red colour that has fascinated people for thousands of years.

The amount of chromium present not only determines the intensity of the colour but also gives ruby one of its most distinctive characteristics: its beautiful red fluorescence. This subtle glow is responsible for the famous "inner fire" seen in many of the world's finest rubies.

Ruby demonstrates one of gemmology's most remarkable lessons—how replacing only a handful of atoms within an otherwise ordinary crystal can create one of the rarest and most valuable gemstones on Earth.

Gemmological Properties

Mineral Species

Ruby is the red gem variety of the mineral species corundum, an aluminium oxide mineral (Al₂O₃). Sapphire is also a variety of corundum, with colours produced by different trace elements or, in the case of colourless sapphire, by the absence of colour-causing impurities.

Because ruby and sapphire are both corundum, they share the same crystal structure, chemical composition, hardness, and most of their physical and optical properties. The primary difference between them is the trace elements incorporated into the crystal during growth, which determine their colour.

Chemical Composition

Chemical Formula: Al₂O₃

Corundum is composed of aluminium and oxygen arranged into an exceptionally strong three-dimensional crystal lattice.

The red colour is produced when small amounts of chromium (Cr³⁺) substitute for aluminium during crystal growth.

This atomic substitution is one of the classic examples of trace elements controlling gemstone colour.

Crystal Structure

Crystal System: Trigonal

Corundum crystallises in the trigonal crystal system, where atoms are arranged in a highly ordered repeating pattern. This efficient packing contributes directly to ruby's exceptional hardness.

Individual crystals commonly develop as:

Hexagonal barrels
Tabular crystals
Bipyramidal crystals

Natural crystal faces may display horizontal growth striations reflecting changing growth conditions within the Earth.

Atomic Structure and Colour

Chromium atoms are slightly different in size and electronic structure from aluminium atoms.

When chromium replaces aluminium inside the lattice, it creates new energy levels that selectively absorb green and blue wavelengths of visible light while transmitting red wavelengths.

This selective absorption creates ruby's characteristic colour.

Small differences in chromium concentration produce noticeable differences between:

Pink sapphire
Ruby
Pigeon Blood Ruby

The exact boundary between pink sapphire and ruby is determined by gem laboratories and market convention rather than nature itself.

Hardness and Durability

Mohs Hardness: 9

Ruby is one of the hardest gemstones used in jewellery.

Only diamond (10) and moissanite (approximately 9.25–9.5) are harder.

High hardness means ruby resists scratching exceptionally well, making it suitable for rings, bracelets and heirloom jewellery worn every day.

Hardness, however, is different from toughness. Although ruby is extremely scratch resistant, it can still chip if struck sharply along areas of structural weakness.

Cleavage and Fracture

Cleavage is the tendency of a mineral to split along specific planes within its crystal structure where the atomic bonds are weaker. These planes are determined by the arrangement of atoms and produce smooth, flat surfaces when the mineral breaks.

Fracture describes the way a mineral breaks when it does not split along cleavage planes. Instead of following planes of weakness, the break passes across the crystal structure, producing irregular or curved surfaces. The type of fracture depends on the mineral's atomic arrangement and the strength of its chemical bonds.

Ruby has no true cleavage, meaning there are no planes of weakness along which it naturally splits. Under certain conditions it may exhibit parting—a break along planes created by deformation or crystal twinning rather than by the crystal structure itself.

When broken, ruby typically displays a conchoidal to uneven fracture, producing curved, shell-like or irregular surfaces.

The absence of true cleavage is one of the reasons ruby is so durable. Unlike gemstones such as diamond or topaz, which have well-developed cleavage and can split if struck in the right direction, ruby is far less prone to cleaving, making it an excellent choice for jewellery that is worn every day.

Specific Gravity

Specific Gravity: ~4.00

Specific gravity measures how heavy a gemstone feels relative to water.

Ruby feels noticeably heavier than quartz and beryl of similar size, making density another useful identification characteristic.

Refractive Index

Refractive Index: 1.760–1.770

Light slows and bends significantly when entering ruby because of its relatively high refractive index.

This contributes to ruby's bright appearance and strong lustre.

For gemmologists, refractive index is one of the fastest and most useful tests for identifying ruby. Ruby has a refractive index of 1.760–1.770, which is generally higher than that of red spinel (typically 1.712–1.736) and red glass (typically 1.50–1.70). Red garnets typically range from 1.720 to 1.830, depending on the garnet species. Pyrope usually has an RI of 1.714–1.742, rhodolite 1.740–1.770, and almandine 1.760–1.830. Because the refractive indices of some red garnets overlap with those of ruby, gemmologists combine RI measurements with other tests, including optic character, pleochroism, ultraviolet fluorescence, spectroscopy, and microscopic examination, to distinguish ruby from garnet with confidence.

Birefringence

Birefringence: 0.008

Because ruby belongs to the trigonal crystal system, light entering the crystal splits into two rays travelling at slightly different speeds.

This property is called birefringence.

Although relatively low, it is measurable using a refractometer and helps distinguish ruby from singly refractive gemstones such as spinel.

Optical Character

Optic Character: Uniaxial Negative

Ruby has one optic axis.

Viewed through specialised gemmological instruments, this behaviour confirms its trigonal crystal structure.

Optical character is one of the fundamental properties used when identifying gemstones.

Polariscope Behaviour

Under a polariscope, ruby behaves as an anisotropic (doubly refractive) gemstone.

As the stone is rotated through 360° between crossed polarising filters, it typically passes through four light positions and four dark (extinction) positions, with extinction occurring approximately every 90°.

Ruby's atoms are arranged in a trigonal crystal structure with one special direction known as the c-axis. The crystal bends light slightly differently depending on the direction it travels. As a result, most light entering the ruby splits into two rays, each experiencing a slightly different refractive index. This effect is called double refraction, or birefringence. As the ruby is rotated in a polariscope, these two rays interact differently with the instrument's polarising filters, causing the gem to alternate between light and dark. This repeating pattern is one of the characteristic optical properties gemmologists use to identify ruby.

This behaviour confirms that ruby is optically anisotropic, a direct consequence of its trigonal crystal structure. It also helps gemmologists distinguish ruby from isotropic gemstones such as garnet and spinel, which remain dark throughout a full rotation under normal conditions because they do not split light into two rays. (Some isotropic gemstones may show anomalous double refraction due to internal strain, producing irregular light and dark patterns rather than the regular extinction seen in ruby.)

Pleochroism

Ruby has a trigonal crystal structure with one special direction called the c-axis. Because of this, light behaves differently depending on the direction it travels through the crystal. Most light entering the crystal splits into two rays with slightly different refractive indices, a property known as double refraction, or birefringence. As the ruby is rotated in a polariscope, these rays interact differently with the polarising filters, producing the repeating pattern of light and dark seen during testing.

Before cutting a ruby, the gem cutter first identifies the c-axis. The stone is then oriented so the table (the large, flat top facet) is at the best angle to the c-axis for that particular crystal. This allows the finished gem to show its richest colour, minimises unwanted optical effects, and helps it appear brighter and more attractive when viewed face-up.

Dispersion

Dispersion: 0.018

Dispersion is the ability of a gemstone to separate white light into spectral colours.

Ruby has relatively low dispersion compared with diamond or moissanite.

Instead of rainbow flashes, ruby's beauty comes primarily from its body colour and chromium fluorescence.

Fluorescence

One of ruby's defining characteristics is its strong red fluorescence.

When exposed to ultraviolet light, chromium atoms absorb invisible UV radiation and re-emit it as visible red light.

Long-wave UV often produces bright fluorescence in chromium-rich rubies, particularly those from marble-hosted deposits such as Myanmar.

Iron-rich rubies from basaltic deposits generally fluoresce much more weakly because iron suppresses chromium fluorescence.

This behaviour provides valuable clues about both origin and composition.

Typical Inclusions

Natural rubies commonly contain:

Rutile silk
Fingerprint inclusions
Mineral crystals
Colour zoning
Healed fractures

Inclusions in Lab Ruby

Laboratory-grown rubies may also contain inclusions or growth features, but these depend on the method used to grow the crystal. Flame fusion rubies may show curved growth lines, while flux-grown, Czochralski-grown and hydrothermal rubies can display subtle growth features or occasional inclusions characteristic of their formation.

Modern crystal-growing technology is advancing rapidly, and today's laboratory-grown rubies are often far cleaner and more refined than the examples traditionally shown in older gemmology textbooks. The outdated perception that synthetic rubies are easily recognised by abundant gas bubbles or obvious growth features is no longer representative of much of the high-quality material available today. In many cases, distinguishing a modern laboratory-grown ruby from a natural one requires careful microscopic examination and advanced gemmological testing.

Lab rubies may also contain internal features, but they reflect the crystal growth process rather than geological formation.

Hydrothermal rubies, which can contain fluid inclusions and growth features that more closely resemble natural crystals.
Some of the advanced technology labs that Gemdrop work with are experimenting with engineered inclusions, growth zoning, fractures, and silk-like features to mimic the appearance of natural ruby under magnification.

Inclusions often help determine whether a ruby is natural or laboratory-grown and provides clues about it's formation.

Diagnostic Features

Ruby can usually be recognised by the combination of:

Rich chromium-red colour
Strong red fluorescence
High refractive index
Moderate pleochroism
Mohs hardness of 9
Trigonal crystal structure
Absence of cleavage

No single property identifies ruby on its own. Instead, gemmologists examine a combination of diagnostic features, including its refractive index, birefringence, pleochroism, fluorescence, crystal structure and inclusions, to identify the gemstone with confidence.

Ruby Treatments

Many mined rubies are treated to improve their appearance. The most common treatment is heat treatment, which can enhance colour and clarity by altering inclusions and redistributing trace elements within the crystal. Lower-quality material may also be fracture-filled with lead glass to improve its apparent transparency, while diffusion treatment is occasionally used to modify colour. These treatments should always be disclosed, as they can significantly affect a ruby's value, durability and care requirements.

Gemdrop lab rubies, by contrast, are not treated after growth. Their colour and clarity are produced during the crystal-growing process itself, so additional enhancement is unnecessary.

Gemdrop® Lab Ruby

One of the remarkable things about ruby is that scientists can recreate the same crystal structure found in nature. Laboratory-grown ruby is genuine corundum with the same chemical composition (Al₂O₃), the same chromium colouring, the same hardness, and the same optical properties as natural ruby. The only difference is where the crystal formed.

Several growth methods are used today.

Flame Fusion (Verneuil Process)

Developed in 1902 by the French chemist Auguste Verneuil, flame fusion was the first commercially successful method for growing ruby. Finely powdered aluminium oxide mixed with chromium oxide is melted in an oxyhydrogen flame reaching temperatures of approximately 2,000–2,200°C. The molten droplets solidify on a rotating support, gradually forming a single crystal known as a boule.

Flame fusion is fast, efficient and capable of producing beautiful gem-quality ruby, although the crystals often contain characteristic curved growth lines that distinguish them from natural stones. Modern flame fusion technology has advanced significantly through improvements in raw material purity, computer-controlled growth, and precision engineering. As a result, today's flame-fusion rubies are often cleaner, more consistent, and of much higher quality than the early synthetic rubies commonly illustrated in older gemmology textbooks.

Czochralski Process

Developed by the Polish chemist Jan Czochralski in 1916, the Czochralski method grows ruby from a carefully controlled molten bath maintained at around 2,050–2,100°C. A small seed crystal is slowly pulled upwards while rotating, allowing a large single crystal to grow atom by atom.

This process can produce exceptionally high optical quality with excellent consistency and is widely used for premium laboratory-grown ruby.

Modern Czochralski systems use advanced computer controls to precisely regulate temperature, rotation speed and pulling rate, producing larger crystals with exceptional optical uniformity, fewer internal defects and highly consistent colour. Improvements in crystal-growth technology continue to increase crystal size and quality, making today's material significantly more refined than early Czochralski-grown gemstones.

Where Ruby is Found Naturally

Natural ruby forms deep within the Earth's crust during high-temperature geological processes, most commonly in metamorphic marble and aluminium-rich igneous rocks.

Important sources include: Myanmar (Burma), Mozambique, Madagascar, Sri Lanka, Thailand, Vietnam, Tanzania, Afghanistan, Pakistan, Greenland.

Each locality tends to produce rubies with distinctive colours, inclusions and fluorescence that experienced gemmologists can often recognise.

Famous Rubies

Sunrise Ruby — A 25.59-carat Burmese ruby that sold for over US$30 million, making it one of the most valuable coloured gemstones ever auctioned.

Rosser Reeves Star Ruby — A 138.7-carat star ruby famous for its exceptionally sharp six-rayed asterism and now housed at the Smithsonian Institution.

The Liberty Bell Ruby — One of the largest carved rubies ever created, weighing over 8,500 carats before it was stolen in 2011.

Why Ruby is Special

Ruby owes its extraordinary beauty to one of the smallest changes found anywhere in nature. Replace just a tiny fraction of the aluminium atoms in colourless corundum with chromium, and the crystal begins to glow with a rich red colour unlike any other gemstone. The same atoms that create its colour also give rise to its remarkable fluorescence, producing the legendary inner fire that has captivated people for centuries.

Ruby reminds us that profound transformation does not always require sweeping change. Sometimes the smallest adjustment, made in exactly the right place, is enough to reveal extraordinary beauty. Nature demonstrates this with a handful of chromium atoms; perhaps our own lives are shaped in much the same way.