Woodbridge Antiques Centre has a wonderful selection of antique and vintage jewellery containing many different types of gemstone. You may expect this article to look at the techniques of some of the most famous jewellers over recent centuries – but no, we are going millions of years further back to look at how nature herself has made the gems.
With just a few exceptions, gems are crystallised minerals which form under the earth’s surface. To understand how they are made we first need to understand something about the makeup of the earth itself. The top layer of the earth is the crust, made of solid rock with a depth that varies between 3 and 25 miles. Under this lies the mantle, which is mainly made up of magma or molten rock. The earth’s crust is divided into tectonic plates which are in constant movement: over millions of years these plates move enough to form and break up continents, oceans and mountain ranges. This is a mechanism that creates huge stresses in the earth’s crust, causing earthquakes and volcanoes, faults and folds. Areas that were once low are uplifted to form mountain ranges, rocks that were deep in the earth are uplifted and erosion brings them to the surface. Other rocks that were once at the surface are “subducted” and taken deep down below the surface where temperature and pressure are huge.
Geologists divide the rocks that make up the earth into three types: sedimentary, igneous and metamorphic. Sedimentary rocks, as the name implies, are those that are formed from deposits of silt, sand, mud or the calcareous remains of marine creatures that build up over millions of years. Sandstone and limestone are examples of sedimentary rocks. Igneous rocks (such as granite) are formed when magma or molten rock is brought near enough to the surface to cool and form solid rock. Metamorphic rocks are formed when existing rocks are subjected to such pressures or temperatures by geological processes that their structure changes. Gneiss, slate and marble are examples of metamorphic rocks.
Gemstones form within these rocks when the correct combinations of mineral content, temperature, pressure and time are present. Many gemstones are formed within rocks that are cooling from a liquid to a solid state, which must happen slowly enough for crystallisation to occur. Most gemstones form within the earth’s crust: only diamonds and peridots are formed in the mantle. Different gemstones form in different rocks:
Gemstones that form in igneous rocks include:
diamond, emerald, amethyst, citrine, beryl, morganite, aquamarine, garnet, moonstone, tanzanite, tourmaline, topaz, spinel and zircon.
Gemstones that form in metamorphic rocks include:
Ruby, sapphire, beryl, jade, lapis lazuli, turquoise, spinel and zircon.
Gemstones that form in sedimentary rocks include:
jasper, malachite, opal and zircon.
Once a gemstone is formed it has, of course, got to be found before it can make its way onto your ring. And it won’t get found 10 miles below the earth’s surface! So, the gems found in jewellery are not only the result of the rare coincidence of conditions that formed them, but are also the result of erosion and other geological processes that bring them to the surface as well as the fortuitous event of someone looking in the right place at the right time. And that is why gems can be so rare and so valuable!