Crystals
Crystals have fascinated people from the earliest times. Their shapes and colours have always been found attractive. People used to believe that crystals had magical powers and could bring good luck. That is why particularly fine crystals, called gemstones, were placed in the crowns of kings and queens in the past.

Crystals form in rock. In fact, most rocks are made from crystals which interlock together to make the rock strong. Deep under the ground is hot molten rock. Above it, is the cool, solid rock called the Earth's crust. In some places the molten rock moves into the crust. Some molten rock even reaches the surface when a volcano erupts. As molten rocks cool either inside the crust or on top of it, crystals form. If the rocks cool slowly, like granite, large crystals form. If the rocks cool quickly, like basalt, tiny crystals form.

Crystals can also form in another way. They can be formed when substances dissolve in water, and then the water evaporates. Sea water contains large amounts of dissolved salt. If a pool of sea water is left on a sunny shore, the water evaporates and the salt forms crystals.

The shape of a crystal is due to the arrangement of particles from which it is made. A crystal begins to form when a few particles become arranged in a special way. They form small sheets, which stick together like a pile of sandwiches. Once the basic shape of the crystal has formed, other particles in the surrounding liquid quickly join onto it. Hundreds may join every moment. At first the crystal is too small to be seen by the naked eye but slowly its flat sides and angular shape can be seen. A crystal needs plenty of space around it so that it can grow perfectly. If it does not get enough space then its shape does not completely form. A crystal also needs plenty of time if it is to become large.

You could see crystals growing out of a liquid for yourself. Dissolve some salt in water then put the liquid in a bowl. Place the bowl in a warm place so the water can evaporate and in a few days you will see crystals growing on the bottom of the bowl.

Are crystals all the same shape?
No. There are seven different shapes of crystal. Some are cubes, some are rectangular blocks, some are rectangular blocks with sloping ends, some are flat sheets, some are shaped like hexagons, some are in the form of two three-sided pyramids stuck together into a long diamond shape while the last group is formed from two four-sided pyramids stuck together to make a short diamond shape. Crystals do not often get the space to form perfect crystals and sometimes their shape is difficult to identify.

Why do crystals have different shapes?
The shape of a crystal is due to the arrangement of the particles and the particles can arrange themselves in different ways. Each way makes a different crystal shape.

Why are crystals different colours?
They have substances in them that give them the colour. For example, sapphires contain iron which makes them yellow. A substance may give different crystals different colours. For example, chromium makes rubies red but makes emeralds green.

Do all crystals shine in the same way?
The way a crystal shines is called its lustre. Some crystals shine like metals while others shine like glass, resin, silk or wax. Some crystals are said to have an earthy or dull lustre.

Are all crystals as hard as each other?
No. The softest crystals are talc. They are so weak that you can grind them to powder with your fingernail. Gypsum, which is a crystal used to make plaster of Paris, can only be scratched by a fingernail. Crystals of calcite are harder. They cannot be scratched by a fingernail but by a copper coin. Quartz is much harder and can scratch glass. Diamond is so hard that it cannot be scratched by anything.

Is glass made of crystal?
No. When crystals are broken up they form smaller crystals with the same shape or bits which have parts that show the same shape. When glass breaks up it forms many pieces, which may look like crystals, but none of them is the same shape. There is a type of glass called crystal glass. This is cut so that it reflects light like some crystals.

Are metals made of crystals?
Yes, they are. In most metals the crystals are too small to be seen by the naked eye. Zinc is unusual in that it forms large crystals when it is coated on steel or iron. If you look at a zinc-coated, or galvanised, bucket you will see the edges of the crystals forming a network across the bucket surface. If you can find a very old brass doorknob, look on its surface to see lines made by the edges of large brass crystals. Many of the crystals in a metal have particles, which are not arranged perfectly. These imperfect arrangements allow the metal to be bent and squashed into sheets.

Are wood and plastic made from crystals?
No. The particles from which they are made form lines like beads in a necklace. These long lines of particles are called molecules. In plastics, the molecules stick to each other to form sheets, blocks or fibres. In wood, the particles form fibres.