Understanding Crystals Doesn't Have to be Complicated
A crystal or crystalline solid is made up of atoms arranged in an orderly repeating pattern that extends in all three spatial dimensions.
The process of crystal formation through crystal growth is called crystallization. As the atoms connect together they form molecules and when molecules come together they form a crystal. Atoms are far too small to be seen with the naked eye, more than one million would easily fit onto a pinhead. Professor Stephen Hawkings once said approximately fourteen billion years ago the entire universe would have been the size of a single atom.
Orderly repeating arrangement of atoms
The specific arrangement of atoms within a crystal is known as a crystal lattice and using a box of eggs makes a good example. All of the eggs are neatly arranged and evenly spaced out in perfect rows and columns to create an orderly repeating arrangement. The size of the formation is dependant on the size of the box. Likewise the size of a crystal is dependant on how much time it has to grow and the amount of space available.
Crystals grow when a liquid cools and solidifies, the longer the cooling process, the more time crystals have to grow. The reason why obsidian which is volcanic glass is not crystalline is because the molten lava cooled so fast that crystals didn't have time to grow. Granite on the other hand is known for its huge crystals because the molten rock cooled very slowly beneath Earth's surface for millions of years.
Any naturally occurring solid described as being crystalline means that it's made up of crystals.
Crystals have smooth surfaces known as faces and straight edges. Whilst some are large enough to be seen with the naked eye an example being quartz, others are microscopic. Some crystals are so small it's even difficult to see them under high magnification. Irrespective of size the one thing that remains unchanged is that crystals within the same mineral varieties share the same crystal structure.
Quartz crystals | Photo courtesy of Stan Celestian (clickable image)
The smallest piece of quartz will be made up of billions of atoms combined to form a crystal lattice. When sodium and chlorine atoms join together in an orderly repeating pattern they form molecules. The molecules then stack together to form a crystal. Once combined the crystals form sodium chloride otherwise known as halite. The mineral halite is rock salt. Another type of crystal is a snowflake. Snow is almost pure crystallized water.
The shape of a crystal is the external formation of the repeating arrangement of atoms. There are just seven different arrangements of crystal symmetry and all known minerals belong to one of these groups.
Naturally occurring solids void of any significant crystal structure are known as mineraloids. These include obsidian, jet which is a type of coal, opal, amber (fossilized tree resin), shungite and moldavite. Although often described as minerals it's not accurate because these materials are not crystalline. For a naturally occurring solid to be classed as a mineral it has to be made up of crystals.