This week’s What’s on the van? comes from Monica Price, Head of Earth Collections.
This time, it’s a special double issue of ‘What’s on the van?’! Two gemstones, that look very different, turn out to be exactly the same mineral, beryl. On the right is a greenish yellow variety of beryl called ‘heliodor’. It gets its name from the Greek word for the sun, and is coloured by a trace of ferric iron. The stone on the left is absolutely colourless because it has no impurities colouring it. This kind is known as ‘goshenite’ after Goshen in Massachusetts, USA, where it was first found.
Beryl is composed of beryllium, aluminium, silicon and oxygen (Be3Al2(SiO3)6 to be precise) and it is not very common. Flawless transparent crystals are rare, and as they are also very hard and durable, they are ideal for cutting into gemstones. Heliodor and goshenite are not often seen in jewellers shops, and nor is the pretty pale pink variety called ‘morganite’. There are two kinds of beryl which are much better known, ‘aquamarine’ and ‘emerald’. Aquamarine is coloured light blue or green by a trace of ferrous iron, while rare and highly prized emerald is vibrant green because it contains a little bit of chromium.
We talk about ‘cutting’ gemstones, but in fact the faces are ground away on a flat fast-revolving flat metal plate charged with abrasive powder, called a lap. The faces are then polished using finer and finer grades of abrasive powder. Each face is cut at a very precise angle to get the maximum amount of sparkle in the gem.
Our heliodor and goshenite gems are from a beautiful collection generously presented to the Museum by Mr Bernie Peel in 2004. You will be able to see lots more cut stones from the Peel collection in the Museum’s gemstone display when the Museum reopens in February 2014.