March 2024 Meeting

At our March meeting, Helen Plumb came to talk to us about ‘Glorious Coloured Gemstones.’ Helen is an internationally renowned gemmologist, as well as having a degree in geology, and has a shop ‘Just Gems’ in Winchcombe selling high-quality diamonds and coloured gemstones, set in fine jewellery. She also sources rare and unusual natural gemstones from around the world for clients who want something particularly special. 

She showed us many slides of various different gemstones and explained that of the 5,000 or so different minerals about 90 are classed as gemstones. They can be organic (plant or animal based, such as ivory, coral or jet) or inorganic (as formed in the Earth’s crust). They are measured in carats (200 milligrams or 0.2 of a gram) which dates back to Roman times. The Romans used the carob seed, which weighs on average the same as the modern carat, to measure the weight of pearls which were highly prized and could only be owned by members of the Imperial family. The Latin name for the carob tree is Ceratonia, which became shortened to carat.

Gemstones are valued for their beauty, durability and rarity, and they have the advantage of being very portable, easy to store and easy to sell. They are also an excellent investment as they only ever increase in value, particularly as they become more rare. Blemishes in gemstones are called inclusions, which all emeralds feature to some degree. Rubies are red, albeit often of a pinkish hue, but sapphires come in every colour, not just blue. The most valuable sapphires are peachy pink and these are favoured in India, where blue stones are considered unlucky.

Diamonds are the hardest gemstones, graded as a 10, whereas rubies and sapphires equal 9 – which is 130,000 times softer than a diamond! Spinel, tourmaline, garnet (quartz) = 8 and jasper, agate and rock crystal = 7, which means they will stand light wear in a dress ring. Tanzanite should never be used in a ring as it is graded as 6½, which is 100,000 times softer than quartz and makes it susceptible to damage whilst being worn.

Rarity makes gemstones more valuable, but with diamonds it is all about size. Less than 5% of gem-quality diamonds weigh more than one carat after being cut. Tiny diamonds go into industry to be used for cutting but, at the other end of the scale, a 10ct diamond is worth around £250,000 and Liz Taylor famously had a 40ct diamond. The largest diamond ever found is the 3106ct Cullinan Diamond discovered in 1905. It was cut down to make the Star of Africa, 8 other large diamonds and 96 smaller stones. The Star of Africa is part of the Crown Jewels, being set in the sceptre with the cross. Diamonds come in all colours too, of which red is the most valuable, and golden brown or yellow perhaps the most well-known. Other gemstones can also look like a coloured diamond.

Helen went on to tell us about jade, which can come in a range of colours beside green, and every Chinese person wants to own jade as they believe it provides protection. The oldest piece of jade known is 8,000 years old! She said that the Chinese also prize red gemstones. The best quality rubies came from Burma and Mozambique but, as rubies became rarer, red spinel came to the fore, but they too are now almost unobtainable. The Black Prince’s Ruby is actually a red spinel, weighing 170ct. Rubies have chromium in them, which is found in Greenland, so it is possible that rubies may well be found there in the future, as well as in Antarctica.

Helen finished her talk by telling us about the Winchcombe meteorite, which landed in the village in February 2021. It was quickly recovered for analysis and found to be 4.6 billion years old and contained amino acids, the building blocks of life. She has since incorporated pieces of the meteorite into cufflinks for one of the residents.           

The evening’s competition for an item of jewellery with a stone was won by Carol Hoare with her coloured stones and Liz Howard was a very close second with her very pretty leaf-shaped brooch containing a variety of coloured stones.