© Oxford University Press 2009 5.6 Genuine gem? Gemstones… …are beautiful …are rare …are...

© Oxford University Press 2009

5.6 Genuine gem?

Gemstones…

…are beautiful

…are rare

…are hard

That’s why they’re expensive!


Amethyst

Amethyst is a gemstone.

5.6 Genuine gem?


Quartz

Amethyst is purple because it contains tiny amounts of manganese.

Amethyst is mainly quartz.

Quartz is silicon dioxide, SiO2

Pure quartz is usually white.

5.6 Genuine gem?


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?


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?


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?


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?



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?



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 hadmixed another metal with it…

5.6 Genuine gem?



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 =volumemass

5.6 Genuine gem?


Density data

Substance Approximate density, in g/cm³

Quartz 2.6

Plastic Around 1.0 for many types of plastic

Amethyst 2.6

5.6 Genuine gem?


Which gem?

About 1000 years ago, Al-Biruni of Persia studied gemstones.

He used this apparatus to measure their densities.

5.6 Genuine gem?


Which gem?

Al-Biruni didn’t only measure density.

He used many other ways to help him identify gems including…

5.6 Genuine gem?


Which gem?

Colour…

5.6 Genuine gem?


Which gem?

Dispersion of light…

5.6 Genuine gem?


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?


Which gem?

Crystal shape…

Diamond and quartz can look similar.But they have different shaped crystals.

5.6 Genuine gem?


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?

© Oxford University Press 2009 5.6 Genuine gem? Gemstones… …are beautiful …are rare …are...

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