Corundum

Characteristics

• Habit - hexagonal, tabular (short prism faces and large pinacoid faces), barrel-shaped or bipyramidal
• Form - hexangonal or trigonal
• Hardness - 9
• Cleavage - none
• Fracture - conchoidal to uneven
• Streak - colourless
• Lustre - adamantine to vitreous
• Colour - red, blue, orange-pink, green, yellow, orange, violet and pink

Corundum features.

Corundum samples from the day workshop.

Introduction

After diamond, corundum is the hardest mineral on earth. Commonly white, grey or brown with variety of gem colours. Is composed of aluminium and oxygen (aluminium oxide). It forms in syenites, some pegmatities and high grade metamorphic rock. It is concentrated in place deposits.

In the beginning of the 19th century they were recognised as having similar properties and only the colours distinguish them. Ruby forms a continuous colour succession with pink sapphire and only stones of the darker hues are called ruby.

Means 'ruby' in Sanskrit (kuruvinda). Rubies and sapphires have been known since antiquity with many set in jewels in royal collections and private museums. Many gemstones came to the EU from the East and were of similar colours so often mixed up. Prefix 'Oriental' was often applied to harder stones. For corundum terms were used like 'Oriental topaz' (yellow stones), 'Oriental amethyst' (violet) or 'Oriental emerald' (green). These terms are meaninglesss but can still be encountered.

Sourced from Ceylon/Sri Lanka in the early modrn period.

Corundum has long been grown in labs. As they have the same chemical composition as natural stones they can be hard to distinguish. They can been clean or have inclusions so need knowledge and experience to know what to look for to ID.

Varieties

Sapphires and rubies have been mined in Sri Lanka for over 3000 years and in India since 300BC.

Ruby

[see paper handouts for more]

Red variety, coloured by corundum. Is pleochroic showing red and orange through a dichroscope. Can show red fluorescence which contributes to its bright colour when viewed in daylight. Best viewed in daylight. Avoid fluorescence.

From the Latin 'ruber' for red. Once considered the most valuable gemstone. Old term of 'pigeon's blook' described a highly prized shade of red.

Stones that are cleaner are more valuable but many contain 'silk' which can enhance appearance. Rare to find one ready for cutting. When mined are momre translucent and have inclusions & fractures therefore often treated to improve. Eg: cavities are fracture-filled with lead glass to improve clarity, blue flashes will indicate this (blue halo round altered rutile inclusions), a composite ruby. Most are sent to the lab to detect treatments and notes on the report.

Rubies are oiled to improve clarity and heated to improve colour, this removes any blue or purple tones and improve the red. High temps can also heal fissures and dissolve inclusions.

Lore: Almost unknown in European jewellery before Medieval period. Then thought to dispel sadness. 16th century writer said rubies gave control over passions, drove out evil thoughts, secured possessions to their rightful owner, reconciled quarrels, brought peace and preserved bodily health and strength ; July birthstone.

Quality factors are colour, clarity and quality of cut. Example scale:

• 1) good strong red with good clarity, lively
• 2) slightly darker, less lively but still good colour and clarity
• 3) lighter stone, more pinkish
• 4) lively but too much pink and borders on being called a 'pink sapphire'
• 5) poor quality, heavily included, translucent to opaque

Sources: Mogok, Myanmar, is the most prestigious location for rubies; Afghanistan; Australia; Kenya; Madagascar; Pakistan; Sri Lanka; Tanzania; Thailand; Vietnam; recently Mozambique.

Examples:

• 'Star ruby' is red with an asterism (optical effect)
• finest is of 1700 carat, from Myanmar, attached to the host of white marble.

City Lit course (June 2026)

Mozambique: a relatively new source of rubies (last 8 years or so); not as expensive as Burmese rubies but is replacing Burma.

Burma: has been mined for hundreds of years and mines are almost depleted; rubies are still considered the best; quite pink; also the Burmese pigeon blood, a natural red, can be £18-20000 per carat; for ID of Burma stones - rutile needles and fluorescence under UV, these give a particular colour pf pinkish red; in Burma, spinel forms close to ruby. In old days didn't know the difference so spinel pops up as a ruby or with rubies (spinel is 1 lower in hardness, has great colour, generally not treated or made as a synthetic so a good stone to buy, spinels generally Burma or Sir Lanka); Monshu mine (2nd best for rubies / Mogo the best) heats its rubies (accepted as a standard treatment).

Thailand: rubies don't fluoresce as you need less iron to flu and these have more iron.

95% of rubies on the market are treated / c.1910 for synthetics used in jewellery / corundum variety so has growth zoning as an ID feature.

Sapphire

[see paper handouts for more]

Lore: began to appear in jewellery c.2000 years ago; popular in Medieval Europe; thought to stimulate purity in thought and action, help wearer bring about peaceful agreements; deliver protection & wellness.

Sources: Beautiful sapphires trad come from Sri Lanka, Myanmar and Kashmir. Sri Lanka known for 'cornflower' - bright, light colour and Myanmar for 'Royal blue' - slightly darker hue. Kashmir discovered after an 1881 landslide, milky velvety texture, legendary status and fetch high prices. More recently from Madag. (1990s), Tanzania, Australia, China, Ethiopia, Thailand, Montana USA.

Quality - quality / colour / cut:

• 1) good, rich blue, lively appearance, good clarity
• 2) slightly lighter but good clarity
• 3) slightly lighter again
• 4) good colour but heavily included, translucent stone
• 5) dark inky stone

Asterism may be seen. Two or three bright bands of light crossing to create star effect - seen when cut as a cabochon, created from light reflection off regular inclusions or internal channels.

Famous: Logan Sapphire at Smithsonian, 422.99 cts, great colour & clarity; Star of India at American Museum of Natural History, 563.35 cts, milky with tiny rutile crystals that give star effect; Lady Di's engagement ring of 1981, 12ct oval blue sapp from Sri Lanka, surrounded by 14 diamonds, then given to Prince William to Catherine Middleton as her engagement ring.

Treatements: Geuda - sapphire with frosty and semi-transp look not suitable for gem cutting. Will be heated so silk (rutile needles) dissolves and becomes blue. May sapp are heated to improve colour; other treatments are the filling of cracks with oil or inward diffusion of chemicals to produce different colours.

Origin determination is difficult. Studying inclusions and chemical composition can produce differing results. The sapphires from Kash/Myan/Sri L/Mada all occur in rocks of different ages, many 100s million years to 10 of million years, so may be able to use this kind of data to determine an origin.

City Lit course (June 2026)

Something in the 5 carat range is looked for and then cut down / c.1910 for synthetics used in jewellery / corundum variety so has angular zoning from growth or colour, as an ID feature.

All colours except red. Includes blue (coloured by iron and titanium) or 'Fancy sapphire' (any other colour such as yellow/green/orange/pink); star sapphire - any colour (not red) with asterism, largest in the world is the Star of India at 563.35cts - cut in Sri Lnak 300 years ago with rutile producing the asterism.

Cornflower blue is the desired colour (Sri Lanka being a good source eg: natural/untreated/c.4 carat would be rare and worth c. £16,000). Colours also teal, roayl (v. popular), china blue (pale).

Price highest for blue sapphire then drops down to pink then fancy sapphires (colourless & other colours, does inc. pink but they are the top of this lot). Heated fancies are brighter.

Colour change sapphire - common sources are Burma and E. Africa; depends on lighting, change between incandescent and fluor (blue to purple / green to purple, red / light pink to deep pink); colour change is more important than clarity, expensive.

Burma: has rutile needle inclusions.

Sri Lanka: Girda heated - a grey stone with rutile needles and appearing as a star sapphire; if heated to 1600-1800 it cleans the stone and becomes transparent and royal blue; largest natural faceted Sri Lankan blue sapp is 478cts and dates from 1921 and sold to Henry Winston.

India: Kashmir, electric blue sapphires are now highly prized as have have been mined out.

Madagascar: gem trade is dodgy, no safety, child labour.

Padparadsha sapphire: rare (means lotus flower), orange/pink, rarist and most expensive variety; from Sri Lanka and Madagascar; £60,000 upwards if it has a Swiss certificate and large enough. One in the class was c.£20,000.

Stuart sapphire: 104 cts, dates from 1214, flaws but fine, set into Queen Vic's crown then moved to the Imperial State crown.

Arab Roots of Gemolgy

Some elements from 'Arab roots of gemolgy: Ahmad ibn Yusuf Al Tifaschi's Best Thoughts on the Best of Stones' (c.1240) trans. Samar Najm Abul Huda. The Scarecrow Press, London, 1998.

Also called Djawhar, Kibrit and Asdjad.

Pliny said that corundum is a gold base stone and all stones except the base metals, started by being corundum, as all base metals started by being gold, but were prevented becoming gold by different factors. So stones may have all started as corundum but were prevented from becoming so by excessive or little / humidity or dryness. Instead they became red or white or green or yellow or other coloured stone that do not melt under fire and can be files (scratched) by iron, while thers are not being called many other names other than corundum.

Corundum is brought from a mine called Sahran on an island behind the island of Sarandeeb. Wind and rain carry the corundum stones from the mountains, then they are picked up as pebbles as alluvial deposits. If the sun shines on this mountain, it sparkles and is called the lightning of the Rahun. This mountain is the one where Adam descended from paradise to the earth.

The best stones being of great luster, colour and brilliance. Rubies show a delicate red colour or deep. The deep ones show a black tinge and the red appears only as veins in the black. A drill is used on it until the defect is removed and the water and mud come out. The stones can contain holes similar to woodworm called Rim, and the inclusion is drilled until removed. If any defects are left in the stone it can split. Most rubies have overwhelming black tinge which comes out with heat treatment. Some sapphires may contain yellow shade which is removed by heat treatment.

Four varieties of corundum - red, yellow, blue, white.

Red (ruby) has four groups:

• wardi, pink rose coloured, ranges from pink to white
• khayri, red colour with purplish shade, or a stronger purple varying in intensity until almost white
• red one of a strong red safflower, ranging from pink to strong safflower red
• bahramani, pure red colour. Bahraman is one of the name of safflower after which this variety of ruby is called. The most valuable.

Yellow sapphire:

• a delicate one, light in colour, lustrous and bright
• khaluqi, colour of tumeric, stronger than the other
• gulnari, colour of pomegranate blossom, the brightest and clearest, the best of the yellows.

Blue sapphire:

• blue
• sky-blue, colour of lapis lzuli
• ultramarine
• zayti, navy-blue a stronger colour

White:

• maha'i, the clearestcand most intense white, the hardest, lower vareity and least valuable of corundum

Most valuable are the red bahramani, the rummani and the wardi of bright, lustrous colur, completely transparent, free of any defect. Defects include cracks and wormwood-like inclusions. Worst colour in ruby is pink with white shade and the porphyry one. Worst in sapphire is with a greyish shade and zayti. Worst in yellow sapphire has a whitish shade.

Corundum cuts all stone similar to diamond but can only be cut by diamond. Cannot be polished on wood from the agigantea tress which is used for polishing all other stones. Can only be polished on brass lap using a mixture of agate stones from Yemen (broken down and burnt, then moistened with water until like glue). When polished it has the highest lustre of all stones. Has good brilliance which no other transparent stone can obtain. Has heavy weight, resistance to fire, can withstand a quick drop in temp. Only the red one remains stable with heat treatment, the other colours become white and calcinate and crack under excessive temp.

Seen in the jewellery market in Cairo stones being sold as genuine blue and yellow sapphire, while in fact they were manufactured, being originally white sapphires. Imitations cannot resist heat treatment without burning while ruby keeps its beauty and colour when heated and improves in appearance.

Believed to protect the wearer from the plague, if worn in a necklace or ring. Helps overcome difficult problems in life. Good for heart palpitations and obsessive anxieties if hung. Lightning never strikes those who wear it in a ring or amulet. Prevents blook clots when hung. Yellow corundum prevents erotic dreams. In India it is said, whoever posesses corrundum stone can attract to them 'powerful beings if these are not carrying such a stone, ont he condition this is done unintentionally and not for experimentation or testing purposes'.

Value and price depend on quality and size of the stone, supply and demand and distance of the mines from the market. This book establishes the price according to the markets in Baghdad and Egypt. 'Ibn Bakhtishew (9th century court physician) mentioned that Djaa 'far al Barmaki's mother gave him a bowl she used to eat from, together with its spoon, both of yellow sapphire, which he sold, and the price he sold the for was the cause of his lifelong richness.'