# © Oxford University Press 2009 5.6 Genuine gem? Gemstones… …are beautiful …are rare …are hard That’s why they’re expensive!

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© Oxford University Press 2009 5.6 Genuine gem? Gemstones… …are beautiful …are rare …are hard That’s why they’re expensive!

© Oxford University Press 2009 Amethyst Amethyst is a gemstone. 5.6 Genuine gem?

© Oxford University Press 2009 Quartz Amethyst is purple because it contains tiny amounts of manganese. Amethyst is mainly quartz. Quartz is silicon dioxide, SiO 2 Pure quartz is usually white. 5.6 Genuine gem?

© Oxford University Press 2009 Cheap imitation? How can we find out if this gemstone is genuine amethyst, or a cheap plastic imitation? For many years, a few unscrupulous dealers have tried to make money from false gems. 5.6 Genuine gem?

© Oxford University Press 2009 Using density Measuring its density will help. What is density? It is the mass of a substance divided by its volume. So a tonne of feathers has a much lower density than a tonne of iron. 5.6 Genuine gem?

© Oxford University Press 2009 Using density The density of genuine amethyst is almost exactly the same as the density of pure quartz. So let’s measure the density of quartz… 5.6 Genuine gem?

© Oxford University Press 2009 Finding the density of quartz - 1 1.How do we find the mass of our stone? (easy!) 2. How do we find the volume of our stone? (not so easy – it’s an irregular shape!) 3. Ideas? 5.6 Genuine gem?

© Oxford University Press 2009 Finding the density of quartz - 2 1.Put the stone in a full beaker of water. 2.Some water is pushed out (displaced) to make space for the stone. The volume of water displaced is the same as the volume of the stone. 5.6 Genuine gem?

© Oxford University Press 2009 Finding the density of quartz - 2 3. The Greek scientist Archimedes was the first scientist to measure volume like this. The story is that he had the idea when he got into his bath and noticed the water rise. He used this method to help him find out whether the king’s crown was pure gold, or whether a dishonest goldsmith had mixed another metal with it… 5.6 Genuine gem?

© Oxford University Press 2009 Finding the density of quartz - 3 1.Find the mass of your stone. 2.Measure the volume of water the stone displaces to find the stone’s volume. 3.Now calculate its density using the following formula density = volume mass 5.6 Genuine gem?

© Oxford University Press 2009 Density data SubstanceApproximate density, in g/cm³ Quartz2.6 PlasticAround 1.0 for many types of plastic Amethyst 2.6 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? About 1000 years ago, Al-Biruni of Persia studied gemstones. He used this apparatus to measure their densities. 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Al-Biruni didn’t only measure density. He used many other ways to help him identify gems including… 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Colour… 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Dispersion of light… 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Hardness… …the ability of a substance to scratch other substances. The softer substance is the one that is scratched. 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Crystal shape… Diamond and quartz can look similar. But they have different shaped crystals. 5.6 Genuine gem?

© Oxford University Press 2009 Which gem? Scientists still use these properties to help them identify gems. They also use more recently developed apparatus and techniques. 5.6 Genuine gem?

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