Quartz is one of the most common minerals in the world. It is a group of minerals that contain a large number of different varieties. There are two main categories of quartz. The first category is macrocrystalline quartz and it includes the gemstones rock crystal, amethyst, citrine, smokey quartz, prasiolite and rose quartz. These are generally transparent gemstones. The second category of quartz is microcrystalline (also called cryptocrystalline) quartz and it contains varieties of chalcedony gemstones, such as agates, carnelian, sard, chrysopase, jasper and bloodstone. Microcrystalline quartz are usually translucent but can be opaque. Jaspers are sometimes described as being a separate category of microcrystalline as they are opaque.
Let’s start with the macrocrystalline quartz. These transparent gemstones have a hardness of between 6 to 7 on the Mohs hardness scale, which means they are hard wearing enough for use in rings and other jewellery.
Probably the most popular quartz in this category is the amethyst, a birthstone for February. It is a purple colour, ranging from a deep purple to a light violet purple. Sometimes a light green gemstone will be described as a green amethyst but it is actually a prasiolite quartz.
The amethyst can contain a number of inclusions such as colour banding, zebra stripes (also described as tiger stripes or fingerprint patterns), and internal fractures. The dark amethyst in the photo above has some large partially healed fractures and some good examples of zebra stripe inclusions.
Amethyst partners well with pearls as you can see with the first antique brooch and the above brooch/pendant.
Citrine, which is one of the November birthstones, is the next popular macrocrystalline quartz. Its colour ranges from a light lemon to a gold. As much of the available citrine comes from amethyst which has undergone a heat treatment, it will contain similar inclusions to those of amethyst. Citrine is used in jewellery as well as in fobs and seals.
An ametrine quartz is a mixture of citrine and amethyst in colour zones. Natural ametrine is found in the Anahi mine in southeastern Bolivia. There were historical stories about the existence of such a bi-coloured gemstone found in the area but it only began to appear commercially from the 1980s onwards.