Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or molecules are completely random. Amorphous materials, such as liquids and glasses, represent an intermediate case, having order over short distances (a few atomic or molecular spacings) but not over longer distances.
The subject of crystals is a source of both pleasure and consternation within the study of gem materials. Crystal perfection and the variety and beauty of crystal form are a delight to gem and mineral collectors, designers, photographers and many others. Yet the fear of scientific complexity in the study of crystal structure and crystallography has turned many people away from a deeper and more fascinating study of gems.
Most solid materials beneath, around and within us are crystalline or at least partially so; nearly all gem and ornamental materials are crystalline. The realm of the crystal is fundamental to gemmology. However, it would be a mistake to think that knowledge of classical crystallography is fundamental to gemmology. It is not. A deeper appreciation of gem materials starts simply with having a reasonable idea of what is meant by the words ‘crystalline’ and ‘crystallinity’.
People who have an interest in crystals, who might need to appreciate crystallinity in order to continue their professions and pursuits, include the collector locating and describing significant gem crystals for an exhibition; a retail jeweller checking a piece of gem-set jewellery brought in for repair; the lapidary faced with an unfamiliar gem material; a jewellery designer working with a group of cut and uncut gems; an artist producing a wall plaque made of various ornamental materials; an earth scientist analysing soil samples in prospecting for gems; a technologist developing new crystal X-ray analysis methods; and all those gemmology students who seek to develop their ability to check gems and ornamental materials in their ambition to advance their future business or hobby. During the study and observation of crystallinity, the significance of non-crystallinity is important in its own right: the lack of crystallinity and the properties of non-crystalline materials are extremely important in gem identification and detection.
