Excerpt from 'Inside China's secret toxic unobtainium mine': Elements that rule the globe (from DailyMail)

Rare-earths are so-called because when they were first discovered in the 19th Century in Sweden, they were believed to be some of the most uncommon elements.

But through further scientific discovery, rare-earths have been found to be relatively abundant in the Earth's crust. However, the high cost of extraction means that only areas with rich deposits are worth exploiting.

Rare-earth metals are typically malleable. They also have high electrical conductivity.

They are often extracted from minerals through a process that involves dissolving elements in different liquids - usually water and a solvent.

There are 17 rare-earths and their purposes include being used in shielding for nuclear reactors, fibre optics, flatscreen displays and earthquake monitoring equipment.

One rare-earth, erbium, acts as a natural amplifier so it is used in fibre-optic cables to boost signals. Terbium generates a change in an electrical circuit when the metal is compressed. That is why it is often found in earthquake monitoring devices for detecting movement along fault-lines.

Sometimes rare-earth elements are combined in alloys to create strong magnets, which are used in wind turbines. The magnets are a crucial part of the generators that convert the rotational motion of the turbine blades into electricity.

The magnets can be made from rare-earths neodymium and samarium, although they are extremely brittle and also vulnerable to corrosion, so are usually plated or coated.

Another rare-earth, dysprosium, is used in many of the advanced electric motors and battery systems in hybrid vehicles because magnets containing the element can be much lighter and therefore more energy efficient.

Dysprosium has a tendency to soak up neutrons - the tiny particles that occur in atoms and are produced in nuclear reactions. Metal rods containing dysprosium are also used in nuclear reactors to control the rate at which neutrons are available. The magnetic properties of dysprosium alloys make them useful in CD players.

Cerium is used in catalytic converters, which cut carbon dioxide emissions from vehicles, while praseodymium creates a yellow colour in ceramics. In alloys, lanthanum softens a metal, making it easier to work with and sometimes more durable too.