Quartz is the biggest gemstone species of all, and one of the most widely used in modern jewelry making. This is an extremely versatile family of stones that includes a wide range of affordable coloured gems, with some remarkable inclusions.
The abundance and beauty of quartz has meant that it has been used as a gemstone since the dawn of history. Beads of quartz have been found in caves in Israel that were occupied between 5,000 and 6,000 years ago, and quartz of different types has been honoured and worshipped through the ages and worn as an amulet to protect against bad luck and poor health.
This section deals with the crystalline variety of quartz, which is found as a single crystal, rather than with cryptocrystalline quartz, which is a giant grouping that appears as a solid mass. The colour range of crystalline quartz is wide, and colour zoning is a common feature that identifies the material. Quartz can vary from transparent to opaque and, although it is not hugely brilliant, it is an attractive stone. It can produce optical effects, such as stars and cat’s-eyes.
Typical inclusions of crystalline quartz are small crystals, needles, delicate veils and internal cracks that give a rainbow effect. Quartz can often include mica crumbs and hematite flakes that flash when the stone is moved in the light; mica produces a silvery flash while hematite causes a red flash.
Treatments and imitations
- Irregularly colored or very pale amethyst is heated to 750-930°F (400-500°C), at which point it changes to a brownish yellow or brownish red colour – a dark citrine. (If the amethyst is heated above 1067°F/575°C it will lose its colour completely.) The majority of dark citrine is actually heated amethyst and it can resemble quality Brazilian topaz. Some of this material is marketed under the name topaz quartz. Deep brown to red quartz is sold as Madeira or Spanish topaz. Amethyst and smoky quartz are also heat treated to create a lemon- lime citrine. Some heated amethyst becomes green and is sold as prasiolite. Tiger’s-eye quartz is heated to produce a red tiger’s-eye.
- It is difficult to detect traces of heat treatment in burned citrines, and the depth and tone of colours produced tend to vary, making them hard to judge. Normally the price will tell you if you have natural material; heat-treated citrine is cheaper. Some heated amethyst does not have a stable colour and, after a couple of years of exposure to the sun, its colour will change and fade.
- Quartz is also coloured with different materials to improve its existing colour or to create imitations of natural stones. Quartz crystals are sometimes immersed in a green plastic dye to imitate emerald; they are then introduced into parcels of rough emerald to serve as the “leading stone”. The leading stone is considered to be the most valuable or desirable stone in the parcel and determines the parcel price. Mixed with other stones, coloured quartz is difficult to spot. Often the pointed termination of the quartz crystal is broken off to leave just the hexagonal crystal body, which then looks similar to an emerald crystal. If quartz has been treated in this manner, you would be able to scratch the coating with a hard metallic point and peel off the colour.
- Quartz is also coloured red to look like ruby and then cut as cabochons in order to make reading the refractive index difficult. Quartz stones are also sometimes boiled with red-dyed oil, to resemble rubies. These are very difficult to spot, but if they are rubbed with acetone or alcohol some of the red dye will come off.
- Aventurine is often sold as green jade to unsuspecting tourists in the Far East. Good glass imitations of aventurine quartz are on the market. • It was discovered that the colour of smoky quartz was influenced by the natural radiation underground. As a result, colourless quartz is now commonly irradiated to turn it a mediocre grayish brown.
On the whole, crystalline quartz is inexpensive. It has always been an affordable stone, accessible to many people who are unable to purchase more valuable gemstones. Siberian amethyst with an intense, even colour is more valuable, as is clear peachy pink rose quartz. Madeira and Palmeira citrines are the most valuable types of crystalline quartz, as they are naturally produced and mined in limited quantities.
Working with quartz
Quartz is a versatile, wearable gem that is available in plentiful supply, in large sizes. It is fairly brilliant and moderately hard; however, care needs to be taken during setting to avoid damaging the stone. It does not have a great lifespan; if you look at antique quartz jewelry you will see a considerable wearing of facet edges and lots of scratches. Rose quartz’s many internal fractures make the stone quite brittle, so included rose quartz is best avoided as it may break when being set or worn. The softness of quartz makes it suitable for engraving and carving.
Care needs to be taken when soldering and polishing as sometimes heat can fade the material. It is also advisable to avoid wearing jewelry in intense sunlight, as this can also make the colour fade.