1. What Exactly are Crystals Rocks Minerals?
2. Definition of a Gemstone
3. Definition of a Crystal
4. Definition of a Rock
5. Definition of a Mineral
What Exactly are Crystals Rocks Minerals?
Crystals rocks and minerals are the building blocks of our planet and mankind has had an inherent attraction to them since the dawn of time. Their use for personal adornment can be traced back at least 82,000 years and throughout history they have remained a constant source of intrigue and fascination. They have been used as tools, been carried as talismans, have been carved into luxury objects and have long been associated with status, wealth and power. The belief that rocks and minerals hold magical powers and healing properties dates back almost to the dawn of time and stories and myths associated with these natural objects have been passed down from one generation to the next. In today's society the practice of using crystals to promote health and well being is widely known as crystal healing and despite being considered by some to be a pseudoscience, it remains hugely popular and is a multi billion dollar industry.
The lure of crystals rocks and minerals is universal and common to almost every human being on the planet yet few people can explain exactly what they are or how they came into existence. We're all familiar with the words rocks, minerals, crystals, gemstones but each word represents something slightly different. Whilst rock tends to be used to describe any geological related material that's hard hence the phrase hard as a rock, it actually describes a material that's made up of more than one mineral. Minerals on the other hand are naturally occurring substances made up of crystals which in turn are made up of atoms.
The study of crystals is known as crystallography, the study of rocks is petrology, minerals is mineralogy and gemstones is 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. Whilst all of these subjects overlap to some degree, they fall under the blanket term of science.
Definition of a Gemstone
When a piece of a rock or mineral is cut and polished (often but not always) to be used in an item of jewellery, it becomes known as a gemstone. Having said that if you cut and polish a piece of granite it doesn't automatically deserve the right to be called a gemstone, well not officially. For a stone to be classified as a gemstone it must include certain attributes some of which include colour, durability, beauty and rarity. Beauty tends to be the most important characteristic but in their own way all four play an important part. Durability encompasses hardness, toughness and stability although the difference between toughness and hardness is often misunderstood.
Most gemstones are cut from crystals.
Definition of a Crystal
A crystal is a solid that's made up of a microscopic highly ordered arrangement of atoms that form a repeating three dimensional pattern known as a crystal lattice. A typical crystal contains billions of atoms and the shape they form is known as the crystal structure whilst their formation and subsequent growth is called crystallization. The crystal structure for each type of mineral is always the same and should it change, the result will be a different mineral. Pyrite and marcasite both have an identical chemical composition in that they’re iron sulphide (iron and sulphur) minerals but the one you end up with depends on the formation of the atoms or the crystal structure that's present. 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 crystalline structure serves as a tool of identification because it's unique and specific to each mineral variety. The crystal structure in a quartz crystal for example will always be exactly the same.
Repeating Arrangement of Atoms in a Crystalline Solid
Crystals are present in many solids but because of their size which can sometimes make them difficult see and the fact they're tightly interlocked, people rarely take much notice of them. Many crystals are so minute they can only be seen through a powerful microscope whilst others are large enough to be seen with the naked eye. An amethyst geode whose crystals are clearly visible is likely to attract far more attention than a mineral whose crystals may not be so easy to see. The long terminated points often present in quartz are an example of large crystals and their external shape like the shape of all crystals, is defined by the crystal structure or shape of the repeating arrangement of atoms inside. Crystals that can only be seen with the aid of powerful magnification are known as cryptocrystalline but the term microcrystalline is also used and specifically means they're microscopic. Macrocrystalline means crystals are large enough to be seen with the naked eye.
The substances which most of us are familiar with exist as a gas, liquid or solid and all three are made up of atoms but the specific arrangement is different for each. In a solid that's crystalline which means it's made up crystals, the atoms form a three dimensional highly ordered repeating pattern but when the pattern is disjointed or random, the solid becomes known as amorphous or non crystalline. The word amorphous comes from Greek and means without definitive shape or form.
Examples of amorphous solids include obsidian which is natural volcanic glass and man-made glass which is produced by heating sand which is silicon dioxide otherwise known as quartz. Non crystalline solids occur when liquid cools so fast that there's no time for crystals to grow. Although man-made glass is amorphous, when silicon dioxide is crystalline it becomes the mineral quartz.
In the diagram below the solid on the left exhibits atoms in a highly ordered repeating arrangement which makes it crystalline so it would be a mineral. Those on the right have no order or structure which makes it amorphous or non crystalline. Naturally occurring non crystalline or amorphous solids are correctly known as mineraloids.
Crystals have time to grow when a liquid cools and solidifies and whilst some grow quickly, others can take thousands or even millions of years. The slower the cooling process, the larger the crystals. A snowflake can be one single crystal or a collection of many. Impurities within a crystal can affect all stages of the crystallization process and are often partly or fully responsible for a change in colour. Heat can also bring about a change in colour by altering the mineral's chemical composition. A crystal may be exposed to heat naturally during its formation or at some point later in its life cycle. Rocks and minerals have been heated by man for thousands of years in order to enhance or change their colour and most of the world's finest gemstones have been heated. The purple colour in the mineral amethyst is the result of impurities of iron whilst citrine can be produced by heating amethyst. A significant quantity of the world's commercial grade citrine is actually heated amethyst because natural citrine is quite rare.
The word "crystals" is often used as a general term for rocks minerals and more specifically tumbled stones particularly when used in holistic therapies such as reiki and crystal healing. It should be remembered however that not all of these naturally occurring solids will have a crystalline structure so may not actually be crystals in the true sense of the word. Obsidian which is a mineraloid is one example and lapis lazuli is another because it's correctly classified as a rock. Lapis is made up of several different minerals so doesn't have a crystalline structure and cannot be classified as a mineral in its own right.
Definition of a Rock
Rocks tend to be made up of at least two different minerals although some are made up of just one. The minerals which bond together to form a rock are all relatively common and are referred to as rock forming minerals. Granite which is one of the most common types of rock is made up of the minerals quartz, mica and feldspar. Marble which is limestone that's been through the process of metamorphism is an example of a rock that's made up of just one mineral with that being calcite. Another example is quartzite which is consists primarily of minute grains of quartz. With rocks being made up minerals as opposed to crystals they do not have a crystal structure.
Rock types are divided into three main groups which are sedimentary, igneous and metamorphic. All three rock types form through different geological processes. One way to explain the nature of a rock is to think of it as a solid mass made up of individual grains all of which have been compacted together and each grain is a separate mineral. In rocks made up of more than one mineral the nature of the grains and the way in which they come together defines the stone's ultimate 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 which leaves room for moisture and air so the rock ends up being 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 and semi molten rock that's below ground) or lava (molten and semi molten rock that's above ground) cools and solidifies. Sedimentary rocks are made up of sediment which has come together under water. Layers of sediment gradually build up, with pressure they become compacted and eventually through cementation form one solid rock but the the process from start to finish takes thousands to millions of years.
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 and once outside of that range they break down and combine with other minerals within the rock to form a new mineral. The process which is known as metamorphism generally takes place deep within Earth.
Definition of a Mineral
A mineral is made up of naturally occurring inorganic solids which means they're void of any living matter. To be correctly described as a mineral a substance must also be crystalline meaning it has a highly ordered repeating arrangement of atoms. Each mineral also has it's own unique chemical composition. Some minerals have the ability to crystallize in more than one way and a different crystal structure creates a different mineral.
Not all mineral-like substances are crystalline, those which lack any significant crystal structure or in other words whose atoms are not ordered or organised in a repeating arrangement, are known as mineraloids. The most common mineraloids include amber which is fossilised tree resin, jet which is a compact form of coal that comes from plant matter, pearl which is formed by shelled molluscs, the tektie known as moldavite, shungite, obsidian and opal.
Most 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 and 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 and include copper, carbon (diamond is a solid form of carbon), gold, titanium and silver.
Native Copper (click to enlarge)
Other chemical elements include manganese, sodium and hydrogen which is the lightest of all elements and formed fourteen billion years ago soon after the big bang along with trace amounts of beryllium. Although an element may be a liquid, solid or gas, bromine and mercury are the only two liquids.
The number of minerals known to man varies from 2,722 to 6,500 depending on the reference that you read. Approximately 150 new varieties are discovered around the world each year.