Not all gems are crystalline, for example opal or obsidian, this lack of a regular structure means that they technically cannot be classed a mineral but are still often collected as such. There are also many gem materials that are of organic origins, such as coral and pearls (which do include some crystalline structure/minerals within the whole) and entirely organic and non-crystalline material such as amber and jet. Rock is the general term for aggregates of minerals and can be uniform in their make-up, such as turquoise, but most often is a mixture of minerals such as unakite.[caption id="attachment_468" align="aligncenter" width="364"] Turquoise and Unakite[/caption]
Most atoms on Earth are bonded (see this week’s extra notes for more details on bonding types) and in general these bonds are formed by interactions of electrons between neighboring atoms. Electrons are one of three major sub-atomic particles, the other two being protons and neutrons. It is electrons that are of greatest interest to the gemmologist due to the bonds they create and their effect on a gem’s colour, hardness, optical effects, crystalline structure, fluorescence and cleavage. Most substances consist of two or more elements bonded together forming what is called a compound, when four oxygen atoms and a silicon atom bond a silicate tetrahedron is formed and this is what quartz is composed of (note that the formula for quartz is SiO2 because each silica atom is shared between two tetrahedron).[caption id="attachment_465" align="aligncenter" width="750"] Silica Tetrahedron[/caption] [caption id="attachment_469" align="aligncenter" width="251"] Quartz Crystal Cluster[/caption]
Though is not always the rule as mentioned above, diamond is an example of a gem mineral of a pure element.Minerals are classified into groups by their general make up, some important groups are as follows: Silicates: This is the most abundant group of minerals and is based on a combination of silicon and oxygen usually with other components. Gems include garnet, peridot, quartz, tourmaline and many more. [caption id="attachment_472" align="aligncenter" width="500"] Cut and Rough Tourmaline[/caption] Oxides: When other elements combine with oxygen they are often called oxides. Gems include ruby/sapphire, alexandrite, hematite and spinel. [caption id="attachment_474" align="aligncenter" width="650"] Rough Ruby and Spinel Crystals Set in Rings[/caption] Carbonates: These are metallic elements bonded with compounds of oxygen and carbon. Gems include calcite, rhodochrosite, malachite and azurite. [caption id="attachment_475" align="aligncenter" width="300"] Chinese Malachite[/caption] Phosphates: When a tetrahedron of phosphorus and oxygen combines with various other compounds. Gems include apatite and turquoise. [caption id="attachment_476" align="aligncenter" width="297"] Rough and Cut Apatite[/caption] In our next article we will look at the nature of crystals.