The fundamental idea is that, all crystals are glasses but all glasses are not crystals. 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 jewelry 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 distinguish between crystal and glass, it is necessary to understand what is a glass and what is a crystal.
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. It 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 it are much smaller than the wavelength of visible rays, which makes them transparent. Obsidian and meteorite glasses are naturally available in the earth and the other types of glasses are made artificially. Some of the other types include silicate glasses, network glasses, sugar glasses, colloidal glasses, etc. A glass would break due to microscopic cracks on its surface. Thermal stress could also cause them to break. What exactly happens during the breaking of glass, is that, the silicon and oxygen bonds of the glass material are broken.
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 can supply electrical voltage when mechanical stress is applied to it. There are various types of crystals such as covalent, metallic, ionic and molecular crystals. Their properties will vary depending on their type. For instance, diamond, graphite, etc., have covalent bonds while table salt has ionic bonds. The number of electrons and protons present in a crystal is balanced. The color will also depend on the color of the light it absorbs. For example, if a crystal absorbs yellow light, it will appear blue, which is called '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 glass is different from crystals.
