As we begin our research into the World of gemstones and crystals let’s take a look at planet Earth and her structure. Mother Earth is a massive ball of rock 7,927 miles in diameter and weighs about 6.6 sextillion tons. A sextillion, for those who don’t know, is the unit number 1 followed by 21 zeros. So, if we were to write down the weight of the Earth on paper it would appear like this – 6,600,000,000,000,000,000,000 tons.
The Earth is composed of a dense inner core which is primarily a solid ball of iron and nickel. Its outer core is made up of iron, nickel, and a convection of liquid metals such as gold, platinum, and uranium etc. These metals can also be found on the surface of the Earth in solid form.
The Earth’s continents are islands of granite rock floating on dense, dark rocks underlying ocean basins that may go down as deep as 600 miles. Minerals of economic value are derived from the lighter granite rocks and rocks sourced from the lighter layer of granite. Mountains form when some of these darker rocks come to the surface of the Earth and are visible.
Minerals are inorganic chemical compounds or elements that occur naturally in the Earth’s crust. Some minerals have a fixed chemical structure and some are a string of related compounds where one metallic element may partly or wholly replace another. For example, jadeite and spodumene are two minerals that are very similar in chemical characteristics and much alike in some of their physical properties, however, both minerals vary greatly in colour and some of their other physical properties.
Elements are a class of substance that cannot be broken down into simpler substances by chemical processes. They are the building blocks of all materials, including rocks and minerals, and they are created by only one type of atom. All atoms of an element have the same number of protons, but, can differ in the number of electrons and neutrons they possess. Elements are defined by the number of protons they contain in the nuclei of their atoms. There are 94 naturally occurring elements on Planet Earth and 24 synthetics, a total of 118 elements.
Source: E. Generalic, https://www.periodni.com/images.html
Geology is the scientific study of the Earth’s structures and materials. Its history and the processes that have caused it to change over time. Divisions within the science of Geology include Mineralogy which is the study of minerals, Petrology the study of rocks, and Crystallography the experimental science and analysis of the seven crystal structures
Gemstones and Crystals are stunning both in natural beauty and proportion. And there appears to be some sort of mathematical formula to how they form in such amazing symmetry. Perfect crystals, however, are very rare and extremely valuable and the majority of the crystals on our planet are characterised by inclusions, blemishes, distortions or some kind of interruption during their development. And some crystals form in clusters or as twins.
There are seven crystal systems in mineral identification:
- Cubic (Isometric) System – Three axes of equal length at right angles to one another, as in a cube. Examples: Diamond, Garnet, Halite, Pyrite.
- Trigonal (Rhombohedral) System – is a system that is sometimes referred to as a subdivision of the Hexagonal system. It has three axes of equal length but no axis is perpendicular to another. All crystal faces are the same size and shape.
- Monoclinic System – Three unequal axes. Two are not at right angles with the third axes making a right angle to the plane of the other two. Examples: Augite, Epidote, Gypsum, Hornblende.
- Orthorhombic System – Crystals with three different length axes all at right angles. Examples: Barite, Celestite, Olivine, Sulfur.
- Tetragonal System – Two axes of equal length and one different in length. All three axes are at right angles to one another. Examples: Cassiterite, Rutile, Zircon.
- Triclinic System: Three unequal axes with none forming at a right angle with any other. Examples: Chalcanthite, Feldspar, Plagioclase, Rhodonite.
The hardness of a mineral is determined by its surface resistance to abrasion. The ease or the difficulty of how one mineral can scratch another.
Around 1810 – 1811, a scientist by the name of Friedrich Mohs created a scale to describe mineral hardness. He assigned the hardest minerals with the highest numbers and the softest minerals with the lowest numbers. The scale goes from 1 to 10, with talc being the softest mineral and diamond the hardest.
Mohs scale of hardness:
Diamond is the hardest mineral on our planet and therefore represented by the highest number 10. But in reality, diamond is so much harder than its closest rival corundum at 9, that a true value for a diamond should be around 40.
Specific Gravity – SG
Specific Gravity is a mineral’s relative weight compared to the weight of the same volume of water at a temperature of 4 °C (39 °F). For example, a corundum crystal weighing 40 grams dry weight, weighs 30 grams when suspended in water, a loss in weight of 10 grams. To work out the specific gravity of corundum we divide the dry weight of 40 grams by the loss in weight of 10 grams and the result is 4. So the specific gravity of corundum is 4.0.
Here are some examples:
Borax 1.7 Sulfur 2.0 Gypsum 2.3 Quartz 2.7 Talc 2.8 Apatite 3.2 Topaz 3.5 Diamond 3.5 Gold 19.3
Cleavage | Parting | Fracture
Cleavage and fracture are definitions that describe the various ways in which a mineral will split or break along a planar surface. When a crystal face breaks away it is called cleavage. The characteristic marks that are left after a mineral breaks or has been chipped are called fractures.
The structure of a mineral’s crystal lattice determines cleavage and its tendency to break along a flat plane. Cleavage planes are caused by the arrangement of weaker bonds among atoms in the crystal lattice. Cleavage planes often have reflective and smooth surfaces, unlike a fracture. Fractures are breaks in a mineral that do not occur along a cleavage plane.
Below are the types of cleavage planes:
- Basal or Pinacoidal — Only one cleavage plane. Examples are graphite and mica. You can peel mica into thin sheets.
- Cubic — Three cleavage planes intersecting at 90 °. Halite is an example of cubic cleavage and when their crystals are broken, they form more cubes.
- Octahedral— Four cleavage planes in a crystal. Fluorite displays perfect octahedral cleavage and it is common for minerals characterised by octahedral cleavage to be semiconductors. Another example of octahedral cleavage is diamond.
- Rhombohedral — Three cleavage planes that intersect at angles other than 90 degrees. An example is calcite.
- Prismatic — Two cleavage planes in a crystal. Spodumene is one example.
- Dodecahedral — Six cleavage planes in a crystal. Sphalerite is an example.
The quality of cleavage:
- Perfect — Smooth and lustrous surfaces. Often seen as parallel sets of straight lines. When a mineral with perfect cleavage cleaves it will not leave any rough surfaces.
- Good — Good cleavage leaves a smooth surface with minor leftovers of rough residual.
- Poor — Smooth crystal edges are not easily identified due to the dominance of a rough surface.
- Indiscernible — Cleavage is so poor that it is hardly recognised. Also referred to as indistinct.
- None — Broken surfaces that are rough and fractured and never display any cleavage.
Parting is similar to cleavage, however, uncommon and can be present in minerals that don’t display cleavage.
There are two causes for parting:
- Two separate pressures push towards the center of a crystal after forming, causing the inside of the crystal to displace evenly on a flat smooth plane.
- Twinning crystals that separate from one another and leave a flat smooth plane.
Terms that describe mineral fractures:
- Conchoidal (shelly) — A smooth curved surface appearing like a semicircular shell. Broken glass is a good example.
- Even (smooth) — A fracture forming a smooth surface.
- Subconchoidal — Somewhere in between conchoidal and even. Smooth with uneven rounded corners.
- Uneven — A rough or irregular surface.
- Hackly (jagged) — Rough and jagged points resembling broken metal.
- Splintery — A fracture that creates elongated splinters.
- Earthy (crumbly) — A fracture of a mineral that crumbles when broken.
Colour is a characteristic used in identifying a mineral and it’s origin, excluding diamonds.