1. Fascinated by Rocks and Minerals
2. Definition of a Gemstone
3. Crystals and Minerals
4. Definition of a Rock
Man's Fascination with Rocks and Minerals
Crystals rocks and minerals are the building blocks of our planet and man has been aware of them since the dawn of time. Samples taken from three geographic regions confirm their use for personal adornment can be traced back approximately 100,000 years. Tests confirmed that beads created from mollusc shells are likely to have been strung in order to be worn.
Rocks used for the purpose of art specifically in Africa have also been dated back 30,000 years.
Throughout history rocks and minerals have been a source of intrigue and fascination. In addition to art and personal adornment, they were used as tools, carried as talismans, carved into luxury objects and have long been associated with status, wealth and power.
The belief these naturally occurring solids hold magical powers or healing properties is well documented. Stories and myths associated with them have been passed down from one generation to the next.
Today rocks and minerals are widely used to promote health and well being. When used for this purpose the word 'crystals' if often used. Although 'crystal healing' as it's known is considered to be a pseudoscience, the practice is hugely popular around the world. In recent years the market for 'crystals' has exploded and it's now a multi billion dollar industry.
The lure of crystals rocks minerals and gemstones is universal and common to every human being on the planet. Despite their widespread use, few people have a good understanding of what each one is. These four words are often used interchangeably but each one represents a naturally occurring material that's slightly different.
Use of the word 'crystals' as a blanket term has caused no end of confusion.
When used correctly the word 'rock' describes a naturally occurring solid made-up of more than one mineral. Rocks tend to be quite hard hence the idiom "hard as a rock".
Minerals which form naturally through geological a process are made up of crystals. A crystal is formed from a highly ordered repeating arrangement of atoms.
The study of crystals is crystallography, rocks is petrology, minerals is mineralogy and gemstones gemmology. Geology is the study of the Earth, its history, the rocks of which it's constructed, their structure, where they came from, how they have changed over time and how they continue to change.
Definition of a Gemstone
When a piece of rock or mineral is cut and polished mainly (but not always) for personal adornment, it becomes known as a gemstone.
For a material to be officially classified as a gemstone it must have certain attributes. The most important is colour although some are colourless. Durability, beauty and rarity are the other primary characteristics that make a gemstone different from an ordinary stone.
Beauty comes down to individual taste which metaphorically means almost any stone that has been cut and polished could be called a gemstone. Coprolite which is fossilised dinosaur faeces is a prime example.
Durability encompasses hardness, toughness and stability although the difference between toughness and hardness is often misunderstood.
Most fine gemstones are cut from crystals.
Crystals and Minerals
A crystal is a naturally occurring inorganic solid made-up of a highly ordered arrangement of atoms. These form a repeating three dimensional rigid pattern known as a crystal lattice. Inorganic means not formed from living matter.
A typical crystal contains billions of atoms and the shape they form is known as the crystal structure. Their formation and subsequent growth is called crystallisation.
The crystal structure for each mineral is always the same and should it change, the result is likely to be a different mineral. Pyrite and marcasite have an identical chemical composition in that they’re both iron sulphide minerals (chemical compound of iron and sulphur). The one that forms depends on the formation of the atoms.
Another example is calcite which has more crystal structures than any other mineral. Its chemical composition is identical to aragonite yet they're two different minerals. A mineral's crystal structure serves as a tool for identification because it's unique and specific to each variety.
The crystal structure in a quartz crystal for example will always be exactly the same.
Repeating Arrangement of Atoms in a Crystalline Solid
Where crystals are present in a naturally occurring solid the material is classed as a mineral. Some crystals are difficult to see either because of their size or because they're tightly interlocked or fused together.
Others can be large enough to be seen with the naked eye. Minerals with large visible crystals such as quartz often attract far more attention than those whose crystals are not so easy to see.
Crystals vary in size from a few millimetres to several metres. Shapes are defined by the crystal structure. Cube shaped crystals in the mineral pyrite are common. In fact cubes are one of the simplest and most common shapes found in crystals and minerals.
Any naturally occurring substance with a mass is made up of matter. The three states of matter are solids, liquids and gases. All three are made up of atoms but the specific arrangement is different for each.
For a solid to be crystalline it must have a highly ordered repeating arrangement of atoms. If the pattern is disjointed or random it becomes known as an amorphous or non crystalline solid. The word amorphous which comes from Greek means 'without definitive shape or form'.
Obsidian which is natural volcanic glass is an example of an amorphous or non crystalline solid. Another is man-made glass which is produced by heating sand. Sand is silicon dioxide which is found most commonly in nature as the mineral quartz. Non crystalline solids are also known as mineraloids.
Crystals grow when liquids cool and solidify. If they cool too quickly there's not enough time for atoms to arrange themselves in an orderly repeating pattern. The solid that subsequently forms will therefore not be crystalline. An example is obsidian which is forms from lava which has cooled rapidly on Earth's surface.
Crystals can also form as a result of precipitation from water. Water can only hold a certain amount of dissolved minerals and salts. As the quantity of minerals increases they can no longer remain dissolved in the water. The particles then come together to form a solid. An example is halite. One of the main uses of this mineral is to flavour food. Halite is better known as rock salt.
Whilst some crystals grow quickly others can take thousands or millions of years. The slower the cooling process the larger the crystals.
Impurities within a crystal will often be partly or fully responsible for a change in colour. Heat is usually a contributing factor because it alters a mineral's chemical composition. Rocks minerals and gemstones have been heated by man for thousands of years.
Minerals are chemical compounds which means they're made up of two or more chemical elements. An example is quartz which is made up of one atom of silicon, two atoms of oxygen. A chemical element contains just one type of atom. Minerals made up of a single element are known as native elements. Examples include copper, carbon, gold, titanium and silver.
Native copper (click to enlarge)
Definition of a Rock
Rocks are usually made up of several different minerals. Approximately 5,000 minerals exist on Earth yet most rocks are formed from a combination of some of the most common. Granite which is a common igneous rock is made up of the rock-forming minerals quartz, mica and feldspar.
Even though the minerals that make up rocks are crystalline, the rock itself is not made-up of crystals. Although often described as a mineral, lapis lazuli is in fact a rock because it's made up of many different minerals.
The three main types of rock are sedimentary, igneous and metamorphic. All three form through different geological processes.
One way to explain the nature and composition of a rock is to think of it as one solid mass made-up of individual grains. Imagine the grains have all been compacted together. Now think of each one as a separate mineral. The nature of each grain and the way it comes together defines the rock's hardness. Rough grains which fit snugly together restrict the amount of room left available for moisture hence the rock will be hard and non porous. Finer grains are likely to have a more rounded shape hence won't lock together as tightly. That leaves more space for moisture and air so the rock will be softer and is more likely to be porous.
All rocks begin life as igneous rocks but over time are altered by natural geological processes. Igneous rocks form when magma (molten rock below the surface) or lava (molten rock above the surface) cools and solidifies.
Sedimentary rocks are made up of sediment which has come together under water. Layers gradually build up over time and with the resulting pressure become compacted together. Through the process of cementation the mass slowly transforms into rock.
An existing rock which undergoes a profound transformation caused by heat or pressure is known as a metamorphic rock. Minerals are only stable within a certain range of temperatures and pressures. Once outside of that range they break down and combine with other minerals within the rock to form a new mineral. The process known as metamorphism generally takes place deep within Earth's crust. Marble is a metamorphic rock that began life as limestone.