Gemology is a pretty well-known term to the general public, but crystallography is one that very few people are familiar with. “I never use that word; I think it’s a weird word,” said Geralyn Sheridan, our artisan jeweler, GIA Graduate Gemologist, and company namesake. Even so, crystallography is a huge piece of gemology. We picked her brain to gain a better understanding.
While gemology is the study of gems, crystallography is the study of crystals and crystal structure. “Gems are crystals – precious crystals –,” Geralyn notes, “but salt and sugar are also crystals.” Crystallography at its core is about anything that crystallizes in nature.
Crystal is defined by Merriam-Webster as “a body that is formed by the solidification of a chemical element, a compound, or a mixture and has a regularly repeating internal arrangement of its atoms and often external plane faces.” Since gemstones are just rare crystals, they actually have more in common with the sugar in your pantry than you would think!
There are seven types of crystal structures: cubic, hexagonal, tetragonal, orthorhombic, trigonal, monoclinic, and triclinic. Diamond and spinel are both cubic and a common natural shape for both is octahedron, which looks like two pyramids stuck together at the bases. Though, spinel can also form flattened crystals, which happens when the pyramids that form an octahedron rotate against each other during growth.
Sapphire and morganite are hexagonal, alexandrite and topaz are orthorhombic, and amethyst and citrine are trigonal. There are more gemstones than we can cover here, but the International Gem Society has put together a handy chart you can explore. Our favorite non-gem crystals, salt and sugar, fall into the same categories as their gemstone brothers and sisters. Just like diamond, salt is cubic, and sugar is monoclinic just like moonstone and jade.
Crystallography and crystal structure are very important for lapidaries. They have to use their knowledge of crystal structure to cut any individual stone into the best version of itself. For example, in sapphires there is often color zoning inside the crystals. This happens because trace minerals color it, so if the mineral composition changes in the ground while the crystal is forming you can have different colors – or even different types of gems – that grow right next to each other.
Crystallography also influences or actually dictates much of what gemologists use for gem identification. “It is used for optics mainly,” Geralyn notes, “If it’s dichroic or trichroic, singly refractive or doubly refractive… crystal structure determines those things.” Crystal structure also greatly affects the refractive index, which is the main thing used to identify gemstones. Crystallography also allows a gemologist to determine if a gem is natural or synthetic, because there are certain types of inclusions that occur naturally and others that are specific to methods of growth within a lab.
Although she calls it a weird word, Geralyn loves everything gemology, which clearly includes crystallography. “I took the crystal part of gemology and actually turned it into a design element,” she says. Geralyn used raw sugar crystals to create the texture in her Sweet Love designs, which were finalists in the Niche Awards and JCK Jewelers’ Choice Awards in 2014.
Weird word or not, crystallography is at the core of Geralyn’s designs and in the heart of our business.