Common Characteristics of Natural Crystals

Unit Cells

• All crystals are comprised of unit cells. According to Haverford College, the constituents of a unit cell---whether they are atoms, molecules or ions---must reflect the empirical formula of the crystal itself. For example, since magnesium dioxide (MnO2) has a ratio of one magnesium atom to two oxygen atoms, a unit cell of MnO2 will maintain that same ratio.

Packing Efficiency

• Packing efficiency refers to the percentage or fraction of a crystal unit cell that is actually comprised of atoms (or molecules or ions). According to New Mexico State University, all unit cells have packing efficiencies that are less than 100 percent, because there will always be some empty space between atoms (due to their spherical nature).

Symmetry

• According to New Mexico State University, the most fundamental principle of a crystal lattice is that it is symmetrical. This symmetry forces unit cells into certain shapes, such as rectangles, squares and hexagons, so that there are no overlapping or leftover spaces. According to Haverford College, crystals also posses translational symmetry, which you can observe at the atomic level. This means you can move crystals in certain directions, at certain distances and they will appear as though they have not moved at all.

Shine

• Many individuals associate crystals with being shiny. According to Haverford College), crystals get their distinctive shine from their incredibly flat outer surfaces called facets, which are strong reflectors of light.

Brittleness

• According to Haverford College, the majority of crystals tend to be more brittle (and thus more prone to cracking) than non-crystalline or amorphous solids, such as plastics. Crystals typically crack along their facets, so any pieces that break off will have surfaces parallel to the original crystal's facets.

Slow-Forming

• Solids are more likely to become crystalline---and form larger crystals---when they are slow-forming, such as from the slow cooling and condensing of a fluid. This is due to the precise order that the atoms must become arranged in, which requires lots of time.