Discover a gem of a read with this practical introduction to stones.
Ronald Louis Bonewitz has written a fresh and insightful gemstone book for Dorling Kindersley’s Nature Guide series. The practical read is designed to give any budding jewellery or gemmologist a basic understanding of stone types, mining and lapidary, as well as some of the folklore surrounding gems.
Gems are recovered in two ways: from the rocks in which they are formed or from weathered rock debris. The first method is called hard-rock mining, while the second is called placer mining, when gems are recovered from a concentration in a stream or beach deposit.
Precious stones have been mined since antiquity. Lapis lazuli was mined in Afghanistan about 7,000 years ago, turquoise mining started on the Sinai Peninsula 5,000 years ago and emeralds were mined in Egypt at the same time.
Ancient mining techniques were quite efficient. Panning and sieving of stream gravels continues to be used today. Digging of decomposed gem veins has changed only in terms of the tools used.
Hard Rock Mining
Unless the gems are highly valuable, their concentration in the rocks in which they formed rarely justifies the extensive and expensive methods required to extract them. Although a large percentage of diamond is recovered from placers, it is also mined from the “pipes” of kimberlite rock in which it formed. Solid kimberlite is mined by the usual hard-rock methods of drilling and lasting. The rock is crushed and the diamonds are extracted. Other examples of gems recovered through hard-rock mining are tourmaline, topaz, emerald, sapphire, ruby, aquamarine, opal, turquoise and lapis lazuli. Most of these are mined on a small scale using basic tools.
Because many gemstones are hard and dense, once released by weathering they can be transported by water to concentrate in river beds, beaches and on the ocean floor. These concentrations are called placer deposits.
Placer mining uses methods that mimic the original creation of the placer. Panning and sieving are the simplest methods. In panning, the lighter material is washed out of the pan, and in sieving, the denser minerals end up in the centre of the sieve and are sorted from the concentrate by hand.
In another method of placer mining, gem-bearing gravel is shovelled into a trough of flowing water with baffles at the bottom. Lighter material is washed away while the denser gemstones are retained by the baffles.
Sorting and Grading
A significant portion of gemstones recovered from mines and placers are not of gem quality. They lack the colour, shape, or clarity to be cut. In the case of some gems, the rejection rate can be as high as 90%. To avoid discarding usable stones or retaining inferior ones, careful sorting and grading is necessary. Preliminary grading and sorting is done at the mine, and then at each stage before the rough reaches the cutter.
The reshaping of natural materials to use them for personal adornment has occurred for millennia. Organic materials were probably the first to be shaped, because they are more easily worked, followed by softer stones and finally hard stones.
There are several reasons for cutting or reshaping stones. Gems are cut to enhance their beauty, increase their value, and from ancient times to the present day, to increase their magical potency.
Some of the earliest written records point to the belief that certain stones have potent supernatural powers. Although such beliefs still exist, gems are today cut principally to enhance their economic value. The value of a finished gem can be many times the value of its rough, and cut stones are far more saleable than roughs.
The term cutting in the context of gemstones is a misnomer. Gemstone roughs may be sawn to remove poor areas, separate a gemmy area from within a larger stone, or create a preliminary shape. The actual cutting of the gem, however, is done through various stages of grinding and polishing. These stages are explained on the right. Some gemstones are not faceted and may be polished or used for engraving.
The cut used on any particular piece of gem rough is determined by a number of factors in combination. Keeping in mind the shape of the rough, gemcutters choose a cut that minimises wastage. The position of flaws, fractures and inclusions also determine the cut, as does a stone’s cleavage. If the stone is pleochroic, it is orientated in a way that shows its best colour. Stones that exhibit stars are cut so that the star is centred in the finished stone.
This Book Club extract was taken from the April issue of Professional Jeweller. To read the issue online, click here.