Minerals and gems are crystals. A crystal can be made of almost anything. Something is called a "crystal" if the atoms or molecules that make up the substance are arranged in a regular, periodic way. This regular arrangement is called a lattice. A common example of a lattice is a carton of eggs. The eggs in an egg carton are arranged in neat rows and columns, regularly spaced. Another example of a lattice is the way that apples are stacked in the produce section of the grocery store.

Some solid substances have their atoms and molecules arranged in lattices, and these solids are called crystals. This happens at a very small scale, so you can't actually see the arrangement of the atoms with the human eye. But it is very much like the examples of the eggs and the apples, with atoms and molecules in place of the eggs and apples. A common crystal is quartz, which is made up of silicon dioxide molecules arranged in a lattice. Another common crystal is table salt, which is made up of sodium and chlorine ions arranged in a lattice. The silicon that computers chips are made of is a crystal (a lattice of silicon atoms) . And diamonds are actually just a bunch of carbon atoms arranged in a crystal lattice!

A crystal is made up of atoms of the same element or atoms of different elements [like silica (Si) or calcium (Ca)], and the atoms have a regular, repeating arrangement. Crystals are very ordered, the arrangement of a certain crystal is always the same. Crystals grow when more layers of the same elements arranged in the same manner are added to the outside of the crystal. Depending on how long the crystal is allowed to grow and how much space it has to grow in, a crystal can be very tiny (short time, no space)or very large (long time, lots of space).

A mineral is a crystalline solid, meaning that minerals grow in this ordered manner. Minerals are naturally occurring and have specific chemical compositions. For example, quartz is always made up of only silicon and oxygen (SiO₂).

A gem is a rare mineral or an unusual form of some mineral that is appreciated for its beauty. Topaz and garnet are minerals that are also gems. Emerald is really the mineral beryl, but it is beryl with a certain impurity that makes it dark green. What I mean by impurity is that sometimes a different element will substitute for a usual element in a mineral's atomic structure. Other gems are hydrated forms of minerals. For example, opal is silica and water.