The Corundum Family Introduction

The Corundum Family Introduction

Written by Sarah Arnold on 12 April 2011

The corundum gemstone family holds two of the most highly sort after and beautiful gemstones - Ruby and Sapphires. Due to this they have kept a high price bracket in relation to other gemstones. However due to their popularity a large majority has been heat treated to emphasise their colour. In the terms of cost a 10carat natural Burmese Ruby is in pare with a natural coloured diamond.

Corundum gemstones have a trigonal crystal formation. Rubies occur as hexagonal prisms, tables and rhombohedrons. Sapphire crystals occur as a barrel-shape, double pointed hexagonal pyramids and tabloid shapes. Corundum is found in igneous and metamorphic rocks and also in alluvial deposits.

Corundum gemstones are sourced in all types of cuts, mainly brilliant, princess, emerald and cabochons. However to increase the intensity of colour some stones are quite deep.

Loose Sapphire and Diamonds

Sources of Sapphires

Blue Sapphires - Africa (Tanzania, Nigeria, Madagascar, Kenya, Malawi) Austria, Brazil, Burma, Pakistan (Kashmir) Sri Lanka, Thailand, USA (Montana)

Pink Sapphires - Madagascar, Burma, Sri Lanka and Tanzania

Yellow/Green/Purple Sapphires - Australia (Queensland and New south Wales) Burma, Sri Lanka, Tanzania, Thailand and USA (Montana)

Colourless Sapphires - Sri Lanka

Colour changing Sapphires - Tanzania

Sapphires and Rubies - Corundum Family

Sapphire Durability - 9 Mohr's Scale very good toughness

Rubies Durability - 9 Mohr's Scale very good toughness

Sapphires

Sapphires are found in larger quantities than rubies due to the colour impurities are more common than rubies.

The most valuable colour is blue. Burmese and Kashmir are the most sought after gemstones. Kashmir sapphires are mined on the border of India and Pakistan, where the gemstones are deep velvet blue that is sometimes described as a 'cornflower' blue.

Burmese sapphires are an intense blue, bright blue (royal blue) with a tint of purple.

Sri Lankan sapphires have risen in popularity and range in mid-blue with a tinge of violet to pastel blue. The colour usually has a irregular colour distribution, which Sri Lankan cutters are aware of by making sure the colour is on the cutlet. When this is done the stone can look transparent when titled on its side.

Thai blue sapphires tend to be quite dark in colour. They are minded in large quantities by the lapidaries who operate in partnership with the Thai miners. Australian sapphire's can have an 'inky' appearance in artificial light, so they are heat treated in Thailand, this sometimes results in a greenish tint to the stone.

African blue sapphires come in a variety of colours. Nigerian stones are often very dark, where as Tanzanian produces high quality coloured sapphires with a velvet blue colour. Montana mine pastel violet-blue to steel blue sapphires. The colour can be irregular with strong zoning and the material can be included. In general American sapphires are sourced quite small, not usually any bigger than 1 carat. Montana sapphires are mostly natural, but due to Western mining techniques are very expensive.

Fancy sapphires are found in any colour other than blue. Pink sapphires has become very popular and appears regular in jewellery pieces. The colour ranges from baby pink to a vibrant bluish pink. Dues to their popularity 'hot' bluish pinks have increased in price to almost equal a mid-quality blue sapphire.

Yellow sapphires are quite common and range from pale yellow to an intense amber, they used to be called 'oriental' topaz. Pure golden coloured sapphires are quite rare, so are usually heat treated to achieve the golden shade.

Green sapphires used to be called 'oriental' Peridot and can have alternating bands of blue and yellow. Unfortunately this colour can be quite flat and sombre in appearance.

Violet or purple sapphires are coloured by vanadium, which occurs in pale violets to deep purple shades. Small purple sapphires are regarded to be of good quality, however larger sapphires can be quite expensive. Purple sapphires should resemble amethyst but with more brilliance.

Padparadscha which is Sinhalese for a type of lotus flower is a rare and very expensive sapphire that shows pink and orange simultaneously. They are sourced from Sri Lanka. Orange sapphires are usually used to fall buyers into thinking they have a Padparadscha stone.

Colourless Sapphires is sometimes used as a cost effective alternative to diamonds. If cut accurately to a brilliant cut, there can be some strong fire and sparkle. White sapphires are sometimes heat treated to achieve a blue colour.

When the rutile needles in a ruby are aligned, asterism occurs in a form of a six-ray star. Ideally star rubies should be a good translucent red rather than the opaque brownish purple colour usually available. The rays should be sharp silvery white lines that extend to the base of the stone. Rubies may also have a cat's eye effect, but this is very rare.