In mineralogy a crystal or crystalline solid is made up of atoms that are arranged in an orderly, repeating pattern which extends in all three spatial dimensions. The process of crystal formation through crystal growth is called crystallization. Crystals are made up of atoms which have a regular repeating arrangement, the atoms can be made up of the same element or different elements. When atoms connect together they form molecules and when molecules come together in a regular repeating pattern they form a crystal. Atoms are far too small to be seen with the naked eye and as an example, more than a million atoms would easily fit onto a pinhead.
The specific arrangement of atoms within a crystal is known as a crystal lattice. A good example is a box of eggs because all of the eggs are neatly arranged in perfect rows and columns. It's an orderly repeating arrangement of eggs and its size is dependant on the size of the box just as the size of a crystal is dependant on how much time it has to grow and how much space is available.
All minerals are crystalline which means they are 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 meaning it's difficult to see them even with the most 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 these solids are not crystalline or in other words, they do not have a crystal structure. The word mineraloid is used to describe a naturally occuring mineral-like substance that's not made up of crystals.