How to Distinguish Between Crystal and Glass

The fundamental idea is that, all crystals are glasses but all glasses are not crystals. The uses of glasses and crystals are numerous. We use crystal wares and glass wares in our houses, mostly for decorative purposes. Different types of spectacle glasses are widely used in our day-to-day life and we can even make jewelries out of crystals. Now, when we look into the differences between glass and crystal, some differences are visible to the naked eye and some can be found through experiments only. But before knowing how to identify glass from crystals, it is necessary to understand what is a glass and what is a crystal. Let's analyze it.

What is a Glass?
Glass is a supercooled liquid that has a static nature, because its molecular structure doesn't change during the process of melting and solidification. Glass is an inorganic material, which is basically made by melting substances and then cooling it. A sudden change in the thermal expansion coefficient and specific heat capacity, specifically during the melting point of the glass material, which is supercooled, becomes a glass. The particles present in the glass are much smaller than the wavelength of visible rays, which makes them transparent. Obsidian and meteorite glasses are the types of glasses that are naturally available in the earth and the other types of glasses are made artificially. Some of the other types of glasses are silicate glasses, network glasses, sugar glasses, colloidal glasses, etc. Due to microscopic cracks on the surface of the glass and thermal stress, glasses break. What exactly happens during the breaking of glass, is that, the silicon and oxygen bonds of the glass material are broken.

What is a Crystal?
Crystals have special electrical conductivity property, which can be used in electronic and communication devices. Crystals such as Quartz and Rochelle salt exhibit piezoelectric effect which means, these crystals can supply electrical voltage when mechanical stress is applied to it. There are different types of crystals, such as covalent crystals, metallic crystals, ionic crystals and molecular crystals. So, depending on the type of crystal, the properties will change. Crystals like diamond, graphite, etc., have covalent bonds and crystals like table salt have ionic bonds. The number of electrons and protons present in a crystal is balanced. Depending on the color of the light absorbed by the crystal, its color appearance will change. For example, if a crystal absorbs yellow light, it will appear blue, which is called as 'complementary color'.

Differences Between Glass and Crystal

Properties	Differences
Lead Content	Glass has less lead content in its composition while crystal has more lead proportion as compared to glass.
Clarity	Crystals are clearer than glasses.
Smoothness	Crystals are smooth and seamless as compared to glasses.
Refractive Index	Refractive index is the ratio of speed of light in vacuum to the speed of light in any medium. Crystals have higher refractive index than glasses.
Formation	Crystals are sometimes formed when heated silica is cooled slowly but when heated silica is cooled rapidly, glass is formed.
Occurrence	Crystals occur naturally in the earth, whereas some glasses are artificially made.
Range order	Depending on how the correlations decay, a material can be classified as having long-range order or short-range order. Crystals have long-range order and glasses have short-range order.
Diffraction	Diffraction is defined as the "apparent bending of waves around small obstacles and the spreading out of waves past small openings". Diffraction takes place when light passes through crystals but not in an ordinary glass.
Molecular Arrangement	The molecules present in glasses are disorderly arranged whereas the crystal molecules are arranged orderly in a regular lattice.

Both glass and crystal have their own applications depending on their pros and cons. Sometimes it is not possible to identify whether it is a glass or crystal, just by looking at it. In such cases, the reliable way to identify it, can be done through examining the lead content in the material. According to European Union standards, materials that contain 4% lead are called glass and those with more than 10% lead, are defined as crystals. Hope this comparison has left no doubt in your minds regarding how glasses are different from crystals.