Why is red corundum called ruby and blue corundum called sapphire?

Trade convention, going back centuries and never rationalised. Every colour of corundum except red is a sapphire, described by colour: pink sapphire, yellow sapphire, green sapphire. Where pink becomes red, and therefore where a sapphire becomes a ruby, differs between markets, which means the same stone can change name and price by crossing a border.

Is my ruby glass-filled?

If it was inexpensive and looks very clean, quite possibly. Under a loupe, look for gas bubbles inside the fractures and a blue or orange flash effect as you rotate the stone. A gemmologist can confirm it quickly. It matters because filled stones are fragile, easily damaged by heat and acid, and worth far less than the price they often fetch.

Is heated sapphire worth less?

Less than an equivalent untreated stone, yes, but heating is so widespread that untreated material with good colour commands a substantial premium and usually a laboratory report. Heating is permanent, stable and disclosed in reputable trade. It is beryllium diffusion and glass filling that change the picture more sharply.

What makes a star sapphire star?

Rutile needles growing in three directions at sixty degrees inside the crystal. Cut as a cabochon with the dome oriented correctly, they reflect light as a six-rayed star that moves with the light source. Synthetic star stones exist and usually show a star that is too perfect and sits too near the surface.

Is corundum safe in an ultrasonic cleaner?

Untreated or simply heated stones, yes. Fracture-filled, diffusion-treated or heavily included stones, no. Since you often cannot tell by eye, warm soapy water and a soft brush is the safe default for anything whose treatment history you do not know.

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Mineral family

Corundum

Two of the four traditional precious stones are the same mineral, and the split between them is a decision made in the gem trade rather than by nature. Corundum's hardness of 9 also gave the world its first industrial abrasive, which is a good reminder that gems are minerals that happened to be pretty.

At a glance

Aluminium oxide, second only to diamond for hardness. Red corundum is called ruby; every other colour is called sapphire. The line is commercial rather than mineralogical.

Keynote: Hardness nine, and a naming split made by traders
Chemistry: Al₂O₃, trigonal
Crystal systems: Trigonal
Mohs range: 9 throughout
Stones here: 2 in the catalogue
Care rule: The toughest daily-wear family in the catalogue, unless the stone is fracture-filled.
Watch for: Glass-filled ruby; undisclosed synthetics; diffusion-treated colour.

Themes

sovereignty judgement durability the long reign hardness what a name decides

What holds the family together

Corundum is Al2O3, aluminium oxide, trigonal, hardness 9. On the Mohs scale that sits between quartz at 7 and diamond at 10, but the scale is ordinal rather than linear: diamond is several times harder than corundum, and corundum is several times harder than quartz.

Pure corundum is colourless. Chromium substituting for aluminium gives red, and red corundum is called ruby. Iron and titanium together give blue, and that stone is called sapphire. So do all the other colours: pink, yellow, green, orange and colourless corundum are all sapphire, with the padparadscha, a pinkish orange, commanding the highest prices among the fancy colours.

The threshold between a pink sapphire and a ruby is a matter of trade convention and varies by market, which is one of the odder facts in gemmology.

Corundum with rutile needles in three directions produces asterism, the six-rayed star seen in star rubies and star sapphires when cut as a cabochon.

How it forms

Corundum forms where aluminium is abundant and silicon is not, since any available silica would form feldspar or mica instead. That means metamorphosed aluminium-rich rocks, marbles, and some alkali basalts that carried crystals up from depth.

The classic marble-hosted rubies of Myanmar's Mogok valley formed when limestone containing traces of chromium was metamorphosed. Sri Lanka has supplied sapphires from alluvial gem gravels for over two thousand years, and Kashmir produced, for a few decades from 1881, the velvety cornflower material that still sets the benchmark price.

Madagascar became a major source from the 1990s, and Montana produces sapphires from both alluvial and hard-rock deposits.

Industrially, corundum is emery, which is corundum mixed with iron oxides, quarried on Naxos since antiquity and used as an abrasive for two and a half thousand years.

The stones of this family

Ruby (Al₂O₃ with chromium, Mohs 9). Red corundum, the ratnaraj or king of gems in Sanskrit, assigned to the Sun in Jyotish. Burmese stones carried into battle were said to be set under the skin.
Sapphire (Al₂O₃ with iron & titanium, Mohs 9). Every colour of corundum except red. Medieval bishops wore it, Ceylon supplied it for two thousand years, and Jyotish treats blue sapphire as the fastest and riskiest of the nine gems.

Trade names and lookalikes

Corundum has the largest treatment and synthetic market of any gem. Heat treatment is applied to a large majority of sapphires and rubies to improve colour and clarity, is permanent and stable, and is accepted by the trade when disclosed.

Beryllium diffusion pushes colour into the surface layer at high temperature and produces intense oranges and yellows. It must be disclosed, and it substantially affects value.

Glass filling is the serious one. Fractured low-grade ruby is flooded with lead glass to make it look solid and transparent. The result is fragile, damaged by household acids and by a jeweller's torch, and worth a small fraction of untreated stone. Under magnification it shows gas bubbles and a distinctive blue flash in the fractures.

Synthetic corundum has been made since Verneuil's flame-fusion process in 1902 and is produced in vast quantity; most inexpensive red or blue stones in mass-market jewellery are synthetic. It is a real material, honest when labelled, and a curved growth banding under magnification usually gives it away.

Care that follows from the structure

This is the most durable family here. Hardness 9, good toughness, no cleavage though it does have parting planes. Untreated corundum survives daily wear in rings better than almost anything except diamond, and it can be cleaned in warm soapy water or, if untreated, in an ultrasonic cleaner.

The exception is any fracture-filled stone. Glass-filled ruby must never see an ultrasonic cleaner, a steamer, a jeweller's torch or acidic household cleaners, including lemon juice and some jewellery dips. The filling degrades and the stone can look visibly damaged.

Diffusion-treated stones have their colour in a surface layer, so a re-polish can remove it. Tell your jeweller what you have before any work is done.

Star stones should be cleaned gently, since the same rutile needles that make the star can reach the surface.

Прашања за размислување

What in my life have I split into two categories that are actually one thing?
Where do I need something that will last rather than something that will impress?
If a name changes the price but not the substance, what am I really buying?

Stones in this family

🔴

Ruby

Red corundum, the king of gems in Sanskrit

🔵

Sapphire

Every other colour, and Saturn's stone

🔴

The red ray

Where ruby sits

🔵

The blue ray

Where the classic sapphire sits

🪨

The gem silicates

The rest of the jeweller's tray

Frequently asked questions

: Trade convention, going back centuries and never rationalised. Every colour of corundum except red is a sapphire, described by colour: pink sapphire, yellow sapphire, green sapphire. Where pink becomes red, and therefore where a sapphire becomes a ruby, differs between markets, which means the same stone can change name and price by crossing a border.
: If it was inexpensive and looks very clean, quite possibly. Under a loupe, look for gas bubbles inside the fractures and a blue or orange flash effect as you rotate the stone. A gemmologist can confirm it quickly. It matters because filled stones are fragile, easily damaged by heat and acid, and worth far less than the price they often fetch.
: Less than an equivalent untreated stone, yes, but heating is so widespread that untreated material with good colour commands a substantial premium and usually a laboratory report. Heating is permanent, stable and disclosed in reputable trade. It is beryllium diffusion and glass filling that change the picture more sharply.
: Rutile needles growing in three directions at sixty degrees inside the crystal. Cut as a cabochon with the dome oriented correctly, they reflect light as a six-rayed star that moves with the light source. Synthetic star stones exist and usually show a star that is too perfect and sits too near the surface.
: Untreated or simply heated stones, yes. Fracture-filled, diffusion-treated or heavily included stones, no. Since you often cannot tell by eye, warm soapy water and a soft brush is the safe default for anything whose treatment history you do not know.

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