Spinel
Characteristics
- Habit - octahedral
- Form - cubic
- Hardness - 7.5 - 8 (quite hard)
- Cleavage - none
- Fracture - concoidal to uneven
- Streak - white
- Lustre - vitreous
- Colour - red, blue, pastel shades, grey, lilac
Introduction
Aluminium & magnesium oxides, can be transparent. Small amounts of chemical elements called chromophores are behind the colour. Red coloured by chromium (same element as with ruby). Rare ones, from Vietnam, are coloured blue by cobalt (like cobalt-blue glassware). Can be found in a white marble matrix and popular with collectors.
Spinel is hard, not as hard as a ruby, and the crystals have an interior pattern of atoms the same in all directions so doesn't display pleochroism (ruby does). Use a dichroscope to separate rubies from spinels. Common imitations include glass and doublet composites and synthetic produced by several methods.
Popular gem as lower in price than ruby but still with a quality of colour. Red spinels known since antiquity but occurs in other colours. Tough and lustrous with good durability and stability. As durable cleaning can be done with all modern methods inc. ultrasonic and steam cleaning.
Sources: Tajikistan (only place consistently producing large & clean stones). Also Asia (Sri Lanka, Myanmar, Vietnam, Afghan, Pakistan, China, Nepal) Africa (Tanzania, Kenya, Mozambique, Madagascar), Mexico (particularly the black spinel) and Russia.
History:
- pre 1920s 'spinel' only used occasionally to describe gems (eg: 16th century Portugal travellers in Asia/now Myanmar).
- 1500-1700 most gem spinels classified as 'balas ruby'. Balas referred to Badakhshan region of East Tajkistan (where you now get large spinels) & Afghanistan.
- 1700-1900 ruby was the name applied to stones of excellent colour. Association of spinel with ruby lasted even after scientists defined them as different species in 18th century.
- 1920+ became associated with synthetic stones. lab grown spinel was produced as a diamond substitute as well as in colours (eg: blue for aquamarine)
The Hope Spinel: square cut, 50.13 carat, formally in collection of the British banker Henry Philip Hope (1774-1839, ie Hope diamond now in Smithsonian, Washington). From Pamir region of Tajikistan and set on a silver and gold brooch with diamonds. Sold 2015 at Bonhams for c. U$1.5 mill.
Mughal: a coveted gemstone; large polished spinels often engraved with the dates and names of the previous owners esp. 16th - 18th centuries; auction at Christies NY 2019 'Maharaja and Mughal Magnificence'.
Black Prince Ruby:
- 1300s the eldest son of King Edward III came into possession of what he thought was a ruby but it is a red spinel.
- apparently worn by Henry V in his crown on top of the helmet at the Battle of Agincourt.
- sold for small sum before being returned to the Crown Jewels after the Reformation
- polished, irregular shaped stone, c.170 carats, originates from Pamir region.
- includes an ancient drill hole to wear as a pendant or turban ornament. Drill hole now has a ruby set in it.
Mahenge:
- 2007 a spinel crystal weighing 52 kg was found in Tanzania.
- Manhenge can refer to this find or allude to spinel quality.
- has fractures & inclusions & displays a vibrant pink-ish red colour.
- pieces were removed and resulted in 1000s of carat of spinel coming to market.
Lore: recently became an alternative birthstone for August; octahedral shape of crystals may be behind the name - Latin, spina = thorn.
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.
Spinel, garnet and tourmaline are similar to corundum. The mode of formation of the three is similar to ruby and are found near one another.
Comes from Balakhshan on the border with China.
Three varieties:
- red one - mu'aqrabi - after the scorpion, the best
- a peridot-green one
- yellow one
Other varieties mentioned by a gem dealer:
- mu'aqrab - deep red
- 'atash - lesser red than mu'aqrab
- inari - colour of the pomegranate, means pom. in Persian
- niaziki - lesser red than inari
- yellow one - similar colour to yellow sapphire
Look like corundum in colour, clarity and brilliance. In general, the value of a good spinel is half that of a good corundum.
