In mineralogy 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, as an example more than a million would easily fit onto a pinhead.   Professor Stephen Hawkings once explained that approximately fourteen billion years ago the entire universe would have been the size of a single atom. 

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 and 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 how much space is available.

Crystals grow when a liquid cools and solidifies so 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 lava cooled so fast that crystals didn't have time to grow.     

Any natural solid that's 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 others are microscopic and some are so small that it's even difficult to see them with a powerful microscope.  Irrespective of size the one thing that remains unchanged is that crystals within the same mineral varieties share the same crystal structure.

The smallest piece of quartz will be made up of billions of atoms stacked together 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.  Those crystals are better known as salt.  Another type of crystal is a snowflake, snow is almost pure crystallised water.

Obsidian, jet, opal, amber, shungite and moldavite are often described as minerals which is incorrect because they are not crystalline.  The word mineraloid is used to describe a naturally occuring mineral-like substance that's not made up of crystals.