A mineral is defined as “a solid inorganic substance of natural occurrence” and they have a defined chemical composition and structure. There are over 3000 known minerals and although there are over 90 separate elements known, just eight of them make up around 99% of the Earth’s crust. As previously mentioned the Earth is in a constantly active state with convection below the crust driving mountain building, volcanoes, earthquakes and more and it is these earth processes along with erosion that create gem deposits.
The main three classes of rock are igneous, metamorphic and sedimentary. Within these groups different processes lead to gems. In igneous processes, magma and lava cools and crystals form (with the size of the crystals depending on the duration of cooling, with longer periods resulting in larger crystals). Gems often associated with this type of rock include peridot, zircon, spinel and sometimes ruby and sapphire. A common igneous rock is granite, made up of primarily quartz, mica and feldspar. This is of little interest for gems, however as magma cools, it does so at different rates. As the more stable minerals cool the remaining magma becomes richer in rare elements and fluids. These late stage melts crystallize to form deposits known as pegmatites, known for their content gems such as beryl, topaz and tourmaline.
Metamorphic rocks have been altered in some way (all three rock types can be subject to metamorphism). This can occur through heat and/or pressure and is categorised into contact metamorphism (eg when magma rises into the crust and interacts with the existing rock) and regional metamorphism (eg changes in great masses of rock due to the extreme heat and pressure present at great depth). Gems commonly associated with this rock type include garnet, jade, ruby/sapphire and ornamentals such as marble and serpentine.
Metamorphic conditions can even bend and fold solid rock like putty.
Lastly there are sedimentary rocks, these are composed of broken down or dissolved components of other rocks. There are only a few gems produced from sedimentary processes, the most notable would be opal, formed as water permeates silica rich rocks and is subsequently trapped and opal mineralises from the solution. Other materials of note are less valuable stones such as calcite and chalcedony. The most important part of sedimentary processes for gemstones is the formation of placer deposits. This is where gem bearing parent rocks have been eroded away, releasing the gem material within. As the broken down rock is continually transported, the heavier/more dense material becomes more concentrated as the lighter less durable material is broken up and washed/moved away. Diamonds, rubies, sapphires, zircons and many other gems are denser than more common minerals like quartz and feldspar hence important deposits are often formed in this way. This is a very historical type of deposit as they are much easier to mine than trying to break up the parent rock, this means that before the time of mechanised mining, these types of deposits were viably and profitably mined.
Rough gems from placer deposits can be easily identified by their smoothed, water worn surfaces,
as you can see from these cuprian tourmalines we found in Mozambique
Our next article will take a brief look at what makes a crystal and explain some of the basic building blocks of a gem.