Wednesday, 23 November 2011

Fade to blue

I was wrapping up a nice Diplomystus for a customer when a woman asked to see a pendant in the cabinet. Riley had stopped in, so he opened the case and handed her the jewellery. I was still talking to the fish-buyer at this point, but could pick up the conversation in the background.

'So -what is this? It's a lovely colour.'
'It's a dyed turquoise from Arizona. They call it turtle turquoise.'
'Dyed? So does it fade?'
'Well, I don't think so.Um. You'd be be better asking Matt, there, I think.'
'This is dyed. Will it fade? What colour did it used to be?'
'We've had some of this material for a few years now, and I've not noticed any fading - it's not likely to fade any quicker than any other stone. It was most likely just very pale turquoise that they might not have had a market for. Quite a lot of the stuff sold as turquoise is really just dyed howlite. But it's pretty stuff, and dying it makes it of some use for jewellery. If you were to leave it out in bright sunlight for a while it might fade, I suppose, like many naturally coloured stones.'
'But will it? Why did they dye it? It's such a lovely colour - is that because of the dye?'
'...I think so. Yes, probably.'
'I'm going to get it. It's lovely. I just hope it's as nice in ten years time.'
'Me too.'

Friday, 18 November 2011

Meteorites and meteowrongs

Most days, a meteorite will fall to Earth somewhere. Almost all of them are stone, with only a small percentage iron-nickel or stony-iron. It's hard to recognise a stony meteorite - with an iron one you can feel the unusual weight, cut it to see the metallic sheen inside, etch it for crystal pattern and so on. With a stony one, it usually just looks like a slightly shiny stone. And there are plenty of them lying around. I'm sure you've noticed. So how do you find a meteorite?

Short of one smashing a hole in your house or flattening your car, chances are you're not likely to come across one, but it's not impossible. It was once thought the chances of finding a meteorite were too low to bother, but one man made it his life's work to change that perception. In 1923 Harvey Nininger, a teacher, saw a fireball and became fascinated with meteorites. Eventually he quit his job to focus on hunting them, and he was offered space in Denver Museum, with his collection on display. He worked out a system. 'Go out and educate the people; tell the people what they're like, offer a bonus if they find any. And in a country where the land is farmed, they will turn these things up. And that's the way I made the collection.' In 1946,  he founded the American Meteorite Museum near Winslow, Arizona, close to Meteor Crater. By this time he'd built up an enormous collection of material, and he began to push for the study of meteorites to be taken more seriously. The museum moved to Sedona after a new highway was built, and when business began to fall away, Nininger sold his collection to the British Museum and the Center for Meteorite Studies at Arizona State University. He was now in a position to spend more time hunting, so went back to his hobby while writing books and giving lectures on the subject. Science owes this man a great deal.

I frequently get people bringing me their 'meteorite' finds. So far there hasn't been a meteorite among them. Pyrite nodules quite often. Industrial slag. A piece of pottery. Some galena and a lot of... well... pebbles. But, you never know. There was a nice story earlier this month about a farmer who in 2006 had found a big lump of pallasite, spectacularly speckled with olivine. He cut a bit off, realised he had something special, and it ended up being recently designated a new find and given its own name - Conception Junction, after the town in Missouri where it was found. So there can be a happy ending.

Wednesday, 2 November 2011

By the light of the sun

Vikings, as everybody knows, got around a bit. By sailing, largely. They are thought to have reached America, long before it was being called that, they made themselves busy round most of the European coastline and reached the Mediterranean, the Black and the Caspian Seas. They knew how to handle a boat.

This all took place before there were magnetic compasses to help them navigate, so it's all the more remarkable. I had a customer come into the shop last week looking for a piece of iolite, a blue-violet variety of cordierite. They wanted it because they knew it as Viking's Compass - apparently it had been used by Vikings to find the sun on overcast days. Iolite is pleochroic; the colour varies as you turn the stone in the light. This happens as light of different polarizations is bent to different degrees by the mineral structure as it passes through. I'd not heard of this use of the stone before, and also hadn't heard of an iolite source in Scandinavia. It's not a hugely rare stone, however, and I mentioned above, these were some well-travelled guys.

In the news today, though, is an article about a cleavage rhomb of Iceland Spar - or optical calcite, above right - found on an Elizabethan ship sunk in 1592. It looks like it had been used as a navigation aid in a manner similar to that mentioned above. This clear form of calcite has a set of recurring planes of weaknesses - cleavages - which cause it to break into distinct rhombic shapes. It's known for its birefringence, where light passing thought the rhomb will produce a double image. Another way to find the sun, by rotating the stone until the images are of the same intensity. This find, though dating to a few hundred years after the Vikings had calmed down somewhat, adds weight to the belief that they used some form of crystal to find the sun and, subsequently, their direction. And it'd have been pretty easy for Vikings to get their hands on samples of Iceland spar. Somewhere or other.