Gem library
Ruby
Red corundum, pigeon's blood
Ruby is red corundum — the same mineral as sapphire, coloured by chromium. At hardness 9 with no cleavage, it is one of the most resistant gems in existence, and the most expensive per carat after coloured diamond.
- Hardness (Mohs)
- 9
- Density
- 3.97 – 4.05
- Crystal system
- Trigonal
- Refractive index
- 1.762 – 1.778
- Cleavage
- None (parting along twin planes)
Mineralogical identity
- Family
- Oxides — corundum group
- Species
- Corundum
- Variety
- Ruby
- Chemical formula
- Al₂O₃ + Cr
- Crystal system
- Trigonal (rhombohedral)
- Classification
- Precious stone
- Rarity
- Very rare
- Indicative price
- €500 – 50,000/ct (unheated Burmese: record above $1M/ct)
Etymology
From the Latin ruber, «red». The word corundum comes from the Sanskrit kuruvinda.
Physical & optical properties
- Hardness (Mohs)
- 9
- Density
- 3.97 – 4.05
- Refractive index
- 1.762 – 1.778
- Birefringence
- 0.008 – 0.010
- Cleavage
- None (parting along twin planes)
- Fracture
- Conchoidal to splintery
- Lustre
- Vitreous to subadamantine
- Diaphaneity
- Transparent to opaque
- UV fluorescence
- Strong red
- Pleochroism
- Weak
- Colours
- red, pinkish red, purple red
Formation, geology & history
Ruby and sapphire, one mineral
Pure corundum is colourless. A trace of chromium makes it red: that is ruby. Iron and titanium make it blue: that is sapphire. Every other shade — pink, yellow, orange, green, violet — is a «fancy» sapphire. The boundary between ruby and pink sapphire is the subject of permanent commercial debate, with considerable financial stakes: Western laboratories require a dominant red saturation, while the Asian market is more permissive. Chromium is also responsible for an intense red fluorescence under UV, which gives Burmese rubies their characteristic glow in full light — an effect visible to the naked eye and highly prized.
Treatments: the real market issue
More than 95 % of market rubies are heated, to 1,400-1,800 °C, to dissolve rutile veils and intensify the colour. This treatment is stable and accepted. Far more problematic is lead-glass filling, applied since the 2000s to heavily fractured rubies of very low quality: the glass fills the cracks, the appearance becomes acceptable, but the stone is fragile, tolerates neither hot setting nor acids, and its real value is marginal. Beryllium lattice diffusion also alters colour at the surface. A laboratory certificate — GRS, SSEF, Gübelin, GIA — is indispensable above a few hundred euros, and the mention «unheated» multiplies value five to tenfold.
Mogok, pigeon's blood, and the world record
The Mogok valley in Burma has supplied the world's purest rubies for more than five centuries: metamorphic marbles poor in iron, which lets the chromium fluorescence express itself fully. The term «pigeon's blood» denotes a pure, slightly bluish red with strong fluorescence. In 2015 the «Sunrise» ruby, 25.59 carats, Burmese and unheated, reached 30.3 million dollars at Sotheby's — more than a million per carat, the absolute record for a coloured gem. Today Mozambique produces most of the world's volume, with very fine stones at Montepuez.
How to recognise it
- Red fluorescence under UVChromium makes ruby glow red under a long-wave UV lamp, often intensely. Garnet, the main look-alike, does not fluoresce.
- Hardness 9It scratches topaz effortlessly. A red stone scratched by topaz is not a ruby.
- Density 4Clearly heavy in the hand. Red glass and almandine garnet are lighter, at 2.5 and 3.8-4.2 respectively.
Imitations, treatments & confusions
- Verneuil synthetic rubyProduced industrially since 1902, chemically identical. Signs: curved growth striae (instead of straight), spherical gas bubbles, over-uniform colour. Production cost: a few euros per carat.
- Lead-glass filled rubySold far below the price of a true ruby but far above its real value. Under a loupe: a network of fractures with blue-orange flash effect, bubbles inside the cracks.
- Garnet and red spinelHistorically confused with ruby: the Black Prince's Ruby in the British crown is a spinel. Garnet does not fluoresce and is isotropic; spinel is also isotropic.
Deposits & origins
Burma (Mogok — the legendary «pigeon's blood», and Mong Hsu), Mozambique (Montepuez, today's leading world deposit), Madagascar, Sri Lanka, Tanzania, Thailand (trading and heating), Afghanistan (Jegdalek), Greenland
Care & precautions
- Warm soapy water and a soft brush: untreated natural ruby is extremely robust.
- Absolute caution if the stone is glass-filled: no ultrasonic cleaning, no steam, no acidic product, no heat. Always warn your jeweller.
- Natural or merely heated rubies tolerate ultrasonic cleaning unless heavily fractured.
- Insensitive to sun, salt and water. Store it apart: at hardness 9 it scratches everything else.
Traditions & crystal healing
- Chakras
- Root · heart
- Zodiac signs
- Aries · Leo · Scorpio
- Element
- Fire
- Month
- July
A stone of life and power in every tradition that has known it: Burmese warriors cut it into their skin to become invulnerable, and Indian tradition calls it ratnaraj, «king of stones». It is credited with vital momentum, passion and courage — but also with needing measured use, so intense is its reputation.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
How do I know if my ruby is synthetic?
Under a loupe, look for curved growth striae and spherical bubbles: those are the marks of the Verneuil process. Natural rubies have mineral inclusions and straight striae. If in doubt, a laboratory decides.
What is a «glass-filled» ruby?
A very low-quality ruby whose fissures have been filled with lead glass. It is fragile, sensitive to acids and heat, and its real value is very low. A certificate must mention it.
Ruby or pink sapphire?
Same mineral, a question of saturation. Western laboratories require a dominant red to speak of ruby; below that it is a pink sapphire — with a price gap that can reach a factor of five.