Quartz!

If you break down the composition of the earth's crust element by element, the two most common present in the rocks of the planet are 1) oxygen (locked up into solid molecules, of course) and 2) silicon.

So would it really surprise anyone that the single most common mineral on the crust of the earth is made of silicon and oxygen? Silicon Dioxide...better known as "quartz."

SiO₂ is, in addition, the basis for many of the rest of the crystalline minerals, and it has a central role in the field of geochemistry.

Let's take the rest of the most common elements on the earth's crust: sodium, potassium, chlorine, aluminum, magnesium, iron, nickel.

When combined with oxygen, the metals form metallic oxides and non-metallic oxides. When the two types of oxides combine with SiO₂, you get two major types of minerals in the earth's crust. When quartz comes together with the non-metallic oxides like potassium oxide, it creates the feldspars, the largest group of minerals on the earth's crust, defined by being light in color. On the other hand, when SiO₂ combines with metallic oxide like ferric oxide or ferrous oxide (the difference between the two is an extra atom of oxygen in the valence) you get the ferromagnesian group of minerals.

The importance of quartz doesn't just stop there though. Water is a near-universal solvent and has a great way of breaking things down. The reason that the ocean is salty is because water dissolves the components of most continental rocks, including those made with sodium and chloride...which then proceed to combine in the water as sodium chloride, or sea salt. What happens to the quartz, though? The oxygen is dissolved in water, but the Silicon atom combines again with some others as a result of combining with water to create the various minerals we call the "clay" groups.

All things considered, quartz is pretty busy!

Quartz also has a really extraordinary property: because its crystalline structure is symmetrical it's piezoelectric, which means that it produces an electrical current when pressure is applied and the shape of the crystal is changed - or alternatively, the shape of the crystal changes when electricity is applied. This is because as it is perfectly symmetrical, it is impossible to separate its changes from pressure. Crystals can exist in one of 32 different forms - 20 of which are non-symmetrical, and quartz lucked out in that it can work this way.

Quartz crystals are generally useful when it's necessary to change energy from one form to another. Microphones turn sound to electrical energy. Though quartz was the first piezoelectric material discovered, it's barely used for that purpose these days: materials with large crystals are common like ceramic oxides (which are usually crystals of things like aluminum oxide).

Crystals are often used to power gadgets in science fiction like laser beams, but their greatest utility is in sound alteration: as quartz crystals work better at high frequencies, they're better at ultrasonics than normal sound waves.