Types of Crystals: A Complete Guide to Nature's Most Ordered Structures

Knowing the different types of crystal is really useful in practical life. It helps us understand why solids like diamonds are ideal for cutting tools or why copper is the default choice for electric wires. From kitchen salt to snow flakes there are different types of crystals with unique properties that are suitable for different purposes. A crystal is a solid whose atoms, molecules and ions are arranged in a highly ordered pattern. Disordered random arrangements of atoms with no long range order are called amorphous solids. 

Table of Contents 

• What Are Crystals?
• Types of Crystals and Their Features
• How Do Different Types of Crystals Form?

What are Crystals

A crystal is one of the most beautiful creations of nature. It is a solid in which atoms, molecules, or ions are perfectly arranged in an orderly repeating three-dimensional pattern. This internal repetitive pattern of crystals is called a crystal lattice which gives each crystal a unique set of properties such as its distinctive shape, brightness, colour, hardness and behaviour. 

Types of Crystals and their Features

Crystals can be classified into different ways: by the types of chemical bond between their constituent particles(particles forming them) and by the shape of their atomic structure (the crystal system). Understanding both of them are necessary as the former tells us what a crystal is made of, the later tells us how it is built. 

Crystals Based on Types of Particles 

Based on the types of chemical bonds between atoms, particles and ions there four types crystals:

• Ionic Crystal
• Metallic Crystal
• Covalent Crystal
• Molecular Crystal

Ionic Crystal: Ionic crystals are formed when the positively charged ions called cations and the negatively charged ions called anions are arranged in alternate layers together with a strong electrostatic force. These crystals have very high melting points. When in solid form these crystals do not conduct electricity but when they are dissolved in water, their ions are free to move and can conduct electricity. One of the classic examples of ionic crystals is the table salt also known as sodium chloride (NaCl). The cubic shape of salt crystals reflects the internal geometric ionic structure in them.

Metallic Crystal: Metallic crystals are made up of the metal atoms surrounded by freely moving electrons that do not belong to any single atom. Due to these delocalised electrons in metallic crystals such as copper can conduct electricity. The unique and remarkable properties of metals make them excellent conductors of heat too. They are shiny, malleable, and ductile. For example, gold, aluminium, silver are metallic crystals.

Covalent Crystal:

In covalent crystals, shared electron pairs hold atoms together across the whole structure of the crystals. The entire crystal is effectively one enormous, continuous molecule which is incredibly powerful. The example of covalent crystal is diamond which is the hardest crystal. The covalent crystals are extremely hard and have a very high melting and boiling point. Covalent crystals are usually poor conductors of electricity as the electrons are attached with their bonds. They have a low density and do not have any shine on the surface.

Molecular Crystal:

In molecular crystals particles are held together with a weak ionic or covalent bond. Due to this property molecular crystals are soft and have a low melting point. For example, medications are given in molecular crystal form to get dissolved and absorbed by our bodies as molecular crystals have low melting points. A few more examples of molecular crystals is ice in which molecules are held together with hydrogen bonds.

Crystals Based on their Shape

Beyond bonding, crystals are also classified by the geometry of their unit cell — the smallest repeating building block of the lattice. There are seven crystal systems, ranging from the most symmetrical to the least:

System	Features	Examples
Cubic (Isometric)	All three axes equal, all angles 90°	Salt, diamond, pyrite, garnet
Tetragonal	Two axes equal, third different, all angles 90°	Zircon, rutile, wulfenite
Hexagonal	Three equal axes at 120°, fourth perpendicular	Quartz, beryl, emerald
Trigonal (Rhombohedral)	Three equal axes, no right angles	Calcite, tourmaline, ruby
Orthorhombic	Three unequal axes, all angles 90°	Topaz, olivine, sulphur
Monoclinic	Three unequal axes, one oblique angle	Gypsum, orthoclase feldspar
Triclinic	All axes unequal, no right angles: lowest symmetry	Turquoise, kyanite, microcline

How do Different Types of Crystals Form

Different types of crystals are formed by repeating geometric patterns during a process known as crystallisation. Cooling, evaporation and precipitation are three different types of mechanism through which crystals are formed. Conditions under which crystal is formed determines its properties such as shape, pattern, size or hardness. For example, some crystals are formed by cooling of magma and hydrothermal solutions such as quartz while others are formed by cooling of magma and metamorphic processes such as gemstones like emerald and garnet.