There are several different types of quartz, including amethyst, citrine, milky quartz, rose quartz, or agate.
Despite the fact that it’s the 2nd most common mineral in the Earth’s continental crust, quartz is a true wonder of nature. Composed of the two most common elements in our planet’s crust (oxygen and silicon), quartz has fascinated both scientists and common people since ancient times. At a molecular level, it forms extremely complicated (and variable) crystals; attempting to describe these crystals would require a considerable amount of detail — you could write books on that subject.
Quartz itself is colorless and transparent (or translucent), but it frequently has impurities that make it even more beautiful. In this post, I’ll be ignoring the more practical uses of this amazing mineral and focus on aesthetics of its natural varieties (I’ll create a post about practical uses a few days, but for now, suffice it to say that quartz is absolutely crucial to the computer industry).
Perhaps the most striking representative of the quartz ‘family’, amethyst has been admired since the dawn of civilization — there are references to ancient Greeks and Romans believing that it prevented you from becoming intoxicated (they even made drinking vessels with it). In fact, the Greek word “amethystos” may be translated as “not drunken”, from Greek a-, “not” + methustos, “intoxicated”.
There is still some debate surrounding its exact chemistry. We know that it has the SiO2 composition (as with every quartz variety), but the impurities that give it its violet color are still a topic of discussion. The most likely possibilities are ferric iron or an interplay of iron and aluminum.
Color can range from a light pinkish to a deep purple, and there is quite a lot of quartz, especially in Brazil, where the biggest mines are (it’s also relatively abundant in South Korea and Austria).
Citrine is the yellow variety of quartz (“citrium” means lemon in Latin), and can be formed by two different mechanisms: in the ferric variety, iron or iron oxide absorbs radiation (from ultraviolet to blue), and the second mechanism, although the specifics are still unclear, is known to be caused by aluminum, lithium or hydrogen inclusions.
It’s easy to guess why the smoky quartz was given this name; the color ranges from brown to black, giving the vague impression that there’s smoke inside the mineral. It can be obtained either naturally or artificially. It also has two varieties: Morion and Cairngorm.
Rose quartz varies in color from light pink to rose red, usually due to titanium or iron (although manganese can also be present). It’s rarely used as a gem because in the vast majority of the cases, it has a lot of impurities; it’s mostly used to carve animals, hearts, etc. However, when clear, it’s quite lovely.
Most varieties of quartz have macroscopic crystals, but chalcedony is different: it’s a so-called cryptocrystalline form, which means that the crystals exist, but they are only visible microscopically. This is generally referred to as microcrystalline quartz in geology.
Because chalcedony has numerous varieties, gem dealers refer to each one by its name. If you plan on getting one, be careful, however: numerous sellers just dye the rock, and it can be difficult to tell by the inexperienced eye because it can have a wide range of colors. Chalcedony also frequently pseudomorphs after organic materials, resulting in petrified wood (in Arizona, a whole forest has been petrified in this way), coral, etc. Types of chalcedony:
So, there you have it — although these varieties are all in the “quartz” family, they look very different!
You have to bear in mind that this is merely a visual separation of the types of quartz and I’ll ask you to excuse me for not using geological criteria that are more relevant — my goal was to show how unbelievably variate a single mineral can be, and raise interest for this matter. If I succeeded (or not) or you want to add something, please contact me.