Sparkling Science

克里斯-伊恩·默瑟现在加入我们in the studio from the Gemological association of Great Britain. He's here to tell us more about it. Hello Ian.

Ian - Hello.

Helen - Thanks for joining us. Now first off what is it that makes a gem a gem and how do we know that one is a gem stone?

Ian - Well, when you look at a gemstone and you think, "That's really beautiful. That really appeals to me and I'll get my husband to spend lots of money on it," well that's one of the attributes of gems. Also, they have to last quite a while, don't they? So they have to be a bit durable or hopefully, very durable. I guess pearls are less durable and diamonds are less durable but they are both gemstones. It's a bit variable.

Helen - It's not a strict definition of a certain chemical compound or anything like that?

Ian - It's not that strict and depends on those particular factors and also it helps if they're rare as well. Of course, it is quite rare to get beautiful big crystals. Therefore that's something else that people will pay for and then value. Also they've got to be acceptable, don't they? They've got to appeal to your fashion and appeal to your community or perhaps not come from living elephants and things like that. How acceptable are they? That really also is a factor. Really, I guess in the end it's something somebody pays a lot of money for.

Helen - Excellent. I have here in front of me in the studio a large lump of, what I think is very beautiful. Chris, I don't know if you agree?

Chris - Wow, is that yours?

Helen - No! It's just something I brought with me(!)

Chris - Is this your engagement ring?

Helen - No, Ian brought this in. It's a lump of - I'll describe it - raw crystals, I suppose. It's about the size of my hand, slightly pale blue in colour with some straight edges. What am I looking at, Ian?

Ian - You're looking at aquamarine crystals. They are beautiful gem quality crystals as grown in the Earth, very hot, underneath an area where there are volcanoes. That's just as it forms, hasn't been cut and polished.

Chris - That's the size of Helen's fist, how much would that be worth? Not that I'm thinking of nicking it or anything...

Ian - I would guess you could spend something like one hundred pounds on a group of crystals like that.

Chris - Why are they one hundred quid but a diamond that size would be unfeasibly expensive?

Ian - It's partly the rarity value that's really, how many diamonds do you get on the Earth's surface? Very few. How many big diamonds? Almost vanishingly few.

Helen - What's this made out of?

Ian - That is aluminium, beryllium silicate. It's got beryllium in it which is a strange, rather poisonous metal but these crystals are not poisonous. It's fairly rare and it's a little bit rarer when it's that beautiful blue. It's rarer still if it's in big crystals which are suitable for cutting. Of course, you can only cut a lovely gemstone out of a lovely crystal. You've gotta start with good to get good.

Chris - Chemically speaking, what actually are gemstones? What chemicals do you find in say, rubies and sapphires and emeralds and things?

Ian - Well, many of them are what we call silicates: a little silicon atom with four great big oxygen atoms around it. If you get those which are four cornered units, tetrahedral, they all link together often with metals and that makes up a nice silicate structure. We think of those as minerals or artificial crystals made as silicates. If those atoms come together really well, perfectly - nice orderly arrangement - then you get a nice crystal. You mentioned ruby and sapphire, those are oxides, they're relatively simple. That's corundum. Ruby and sapphire are both the same mineral, called corundum. If you have non-gem quality corundum as a sort of sand that's what many people think of as emery which is use for grinding.

Chris - Sand paper?

Ian - Yes, emery paper.

Chris - It's aluminium, isn't it?

Ian - That's aluminium oxide, yes.

Chris - Why is a ruby such a gorgeous red colour and it's aluminium oxide? A sapphire is that gorgeous blue colour and it's aluminium oxide. What's going on?

Ian - What's going on is impurities. You might think, "Well, how can you call it impure if it's so beautiful?" Well, they are metals which get trapped into that structure of aluminium oxide. In ruby it's chromium. In blue sapphire it's iron and titanium and there are many colours of sapphire, in fact. Many people don't realise sapphire could be any colour.

Helen - I've got a lovely blue one actually. I've got four on my finger which I rather like. You mention volcanoes. Is that where we find all of these gemstones? Is that where they're all formed?

Ian - Many are in volcanic districts. Strangely, those beautiful aquamarine crystals occurred in pockets around granite and granite when it's molten is a bit like porridge. It works its way up towards the Earth's surface. If it crystallises out nice and quickly around the edge of the granite then we've get those lovely big crystals. If it reaches the Earth's surface it forms terrible volcanoes, the most awful dangerous volcanoes ever. Luckily, most of it doesn't get to the surface. The stuff that stays down there might form gemstones.

克里斯,我们怎么知道在哪里寻找不同gemstones? In other words, because they require different conditions - quite specialist conditions to form - does that mean that there are hotspots for different types of gemstones around the planet's surface?

Ian - There are hotspots. Yes. The type of deposit that forms that aquamarine is called a pegmatite and those are prevalent in certain places such as Madagascar, the Ukraine, Brazil, certain states of America and where there are past or present volcanoes.

Chris - If you haven't got a volcanoes there now but you did in the past, that means there presumably a hotspot there for finding things because those conditions exited there once if not today.

Ian - If there's the right type of rock, right type of volcanic province, yes. That's the place that geologists or gem prospectors are going to look. They're clues.

Helen - You touched already a little bit on rarity and what it is that we like. Am I right that in fact engagement rings: that rubies used to be the ones that people wanted because red colour was romantic, it was the colour of roses and hearts and love and things? It was only later that we were persuade that diamonds were a girl's best friend? I don't know if that's a story I picked up from somewhere.

Ian - Well, my wife's engagement ring is ruby. What can I say?

海伦——精彩。

Chris - What does a gemologist give to his wife on their engagement? Now there's a question!