I’ve realized that the terminology used in my posts may not always be completely beginner-friendly. So what is geology, and what does it all mean? That’s what I’m here to explain today. Enjoy these brief definitions, categorized carefully, that you can always come back to. (I know this won’t get as much traction as my usual rock talks, but it’s important knowledge that has taken me upwards of ten hours to compile in a digestible way, so all interactions are appreciated!) Regular rock talks will resume tomorrow as usual.
General Definitions:
Geology: A scientific study of the Earth, its composition, and processes causing changes within the Earth. Subcategories include: palaeontology (study of fossils), mineralogy (study of minerals), and structural geology (study of formed rocks).
Minerals: Naturally occurring, inorganic solid substances with a set chemical composition and crystal structure.
Rocks: Naturally occurring solid substances composed of more than one mineral.
Geological Processes:
Erosion: The gradual weathering and transportation of rock and soil by natural means (wind, water, or ice).
Lava: Molten rock erupted onto the Earth’s surface.
Magma: Molten rock located beneath the Earth’s surface.
Conglomerate: Various rocks and minerals that have become cemented together.
Tectonic Plates: Segments within the Earth’s crust, in constant movement.
Folds: The bending of rock layers caused by compression. Often occurring when tectonic plates collide.
Anticline: An upward arching fold in rock layers.
Caldera: A large crater formed by a volcanic eruption or collapse.
Basin: A large depression within the earth’s surface.
Cave: A natural underground opening in rock.
Fault: A fracture (or area of fractures) between two sections of rock, where the blocks have moved relative to one another.
Mountains: An area of high ground created by the gradual movement of tectonic plates.
Landforms: Natural features of the earth’s surface.
Identification:
The Moh’s Hardness Scale: A universally used tool describing how hard or soft a mineral is. With 10 being the strongest (diamond), and 1 being the weakest (talc). Often used to identify specific minerals. (See chart below.)
Crystal Habit: The external characteristics making up the shape of a crystal formation. Often used to identify specific minerals.
Cleavage: The tendency of crystals to split along planes, creating slightly weaker areas of crystal. Often creating visible folds of crystal, (such as mica). Often used to identify specific minerals.
Fracture: How the surface of a rock or mineral appears when it is broken. Often used to identify specific minerals.
Lustre: How a rock or mineral reacts to light. Can be earthy, metallic, greasy, silky, or vitreous (glassy). Often used to identify specific minerals.
Diaphaneity: How light is able to pass through a mineral or rock (transparency/translucency). Often used to identify specific minerals.
Specific Gravity: AKA relative density, the ratio of density of a specific substance. Solids and liquids almost always use water as the respective reference material. Less than 1: material is less dense than the reference; greater than 1: material is more dense than the reference material. Often used to identify specific minerals.
Refraction index: How much the path of light is refracted (bent) or reflected by a material. Often used to identify specific minerals.
Chemical Formulas: The molecular composition of a rock or mineral, causing different visual and crystalline characteristics. Often used to identify specific minerals.
Types of Rocks:
Igneous: Formed from the heating and cooling of molten rock (magma). Solidifies into Igneous rocks, ie. basalt, granite.
Sedimentary: Formed from the accumulation and cementation of various sediments. ie. shale, sandstone.
Metamorphic: Formed from existing rocks that are changed by heat, pressure, or other chemical reactions. ie. marble.
Wulfenite is an ore of both lead and molybdenum, commonly found in oxidized hydrothermal lead deposits. Wulfenite has a hardness of approximately 3.0 on the Moh’s scale. It was originally discovered in 1845 in Austria, but has since been found in various deposits worldwide. Wulfenite has since been dubbed the official state mineral of Arizona, USA, due to its frequency within the area. Wulfenite is often found throughout mines in the USA, Mexico, Slovenia, and the UK. A pure wulfenite specimen is colourless, but it is very rare to find. Due to trace amounts of other substances, wulfenite can be found in any shade from red to green.
Cummingtonite is a magnesium dominant compound classified as a monoclinic amphibole (referring to needle-like crystalline formations). The mineral was originally found in 1824 in Cummington, USA, but has been a frequent topic of discussion for geologists as recent as 1978, when chemical analyses were (officially) conducted and published. Cummingtonite can be found in shades of dark green, brown, or grey, as well as colourless. These specimens can be extracted from mines in the United States, Scotland, South Africa, Sweden, Japan, Norway, and New Zealand. In order for this mineral to form, it requires a certain environment of magnesium-abundant rocks under significant pressure, causing it to recrystallize. It is often found amongst other metamorphic minerals, like talc or serpentine. Cummingtonite has a hardness of approximately 5.0-6.0 on the Moh’s scale.
Shattuckite is a blue copper silicate mineral, originally discovered in the Shattuck copper mine around Bisbee, Arizona in 1915. It has a hardness of about 3.5 on the Moh’s scale, and can be found in various shades of blue or turquoise. Fun fact: Shattuckite found in the Shattuck mine can form pseudomorphs (a replacement within crystalline structures between two species) after malachite. Shattuckite can occasionally be used as a gemstone, however it is not ideal due to its relatively brittle and soft nature. Shattuckite is relatively rare, since it can be found primarily in the southern United States, with other mines in Mexico, The Democratic Republic of the Congo, Namibia, and Greece.
Stay tuned for another rock talk!
I’d like to apologize for the unprompted hiatus, life got very busy and I haven’t had much free time. But we’re back, baby! More Rock Talks coming soon.
Aegirine is a dark green to brown sodium iron silicate mineral, and is part of the clinopyroxine inosilicate group. It is often found within alkalic igneous rocks or pegmatites in the Northern hemisphere (Canada, Norway, Russia, and the USA, to name a few), as well as Nigeria. It has an approximate hardness of 5.0-6.0 on the Mohs scale. Aegirine is occasionally used as a faceting gemstone. Fun fact: Aegirine was named after the Norse god of the sea, Ægir. It is also referred to as acmite, a Greek name synonymous to “edged/pointed stone” in reference to the typical pointed formation of these minerals.
Dumortierite is a violet to blue-green or brown mineral created by orthorhombic formations of aluminum boro-silicate. It can be found in mines across Peru, Austria, Brazil, Canada, Poland, France, Italy, Madagascar, the USA, Norway, Sri Lanka, Russia, Indonesia, and Namibia. Dumortierite is often used in the manufacturing of porcelain, and is frequently mistaken for sodalite* or lapis lazuli (see post on lapis lazuli here) Dumortierite has an approximate hardness of 7.0 on the Mohs scale, and can be polished and faceted. High quality faceted dumortierite can cost between $26-$90 USD per carat.
*Detailed information about this mineral coming soon.
Emerald is a green gemstone variety of beryl. The mineral gets its vibrant green colour from trace amounts of elements vanadium or chromium. Notable emerald mines can be found in Brazil, Columbia, Zambia, Zimbabwe, Russia, Pakistan, Madegascar, and Afghanistan. The largest single-crystal emerald ever found was discovered in Brazil in 2001, embedded in the magnificent Bahia Emerald. The Bahia Emerald is over 180,000 carats, weighing over 381kg (840 pounds), and has an estimated value of up to $940 million USD. This stone has a vast history, notably being smuggled to the United States in 2005, causing an intense "custody battle" over the gemstone. Emerald has an average price of about $169 USD per carat, and is among the most sought after gemstones for faceting. With a hardness of about 7.5-8.0 on the Mohs scale, it is a great choice for a bold statement or simple jewelry piece.
Moganite is a tectosilicate mineral, originally found in Mogán, Canary Islands. Moganite is usually grey in colour. In 1994 the International Mineralogical Association (IMA) declassified moganite as an official mineral, since it was virtually impossible to differentiate from quartz. In 1999, moganite was reclassified as a mineral species by the IMA. Moganite has the same chemical composition as quartz (SiO2), but forms in a different structure. Moganite can be found across European countries, as well as the USA and India. It has a usual hardness of 6.0 on the Mohs scale, and can be utilized much the same as quartz.
Fluorite is the mineral form of Calcium Fluoride, and is classified as a halide mineral. Fluorite can be found in many rainbow colours, as well as black, brown, white, and pink. Pure fluorite, with no other inhibiting minerals, is colourless. Fluorite is often used for ornamental (jewelry) and industrial (production of some glass, hydrofluoric acid) purposes. The first evidence of fluorite's existence is mentioned in a text from 1530 by Georgius Agricola. Fluorite has properties of both a thermoluminescence and fluorescent glow, the latter of which was actually named after the mineral. Fluorite mines can be found in Canada, Rwanda, China, the UK, South Africa, Mongolia, Argentina, Tanzania, Germany, and Namibia. Fluorite is about a 4.0 on the Mohs hardness scale. Fluorite is alternatively referred to as Fluorspar.
Ammonite is a preserved shell belonging to an Ammolite or other creature belonging to the subclass Ammonoidea. These fossils are the remains of an extinct marine cephalopod (mollusc) from the Jurassic period (about 200 million years ago) to the late Cretaceous period (about 66 million years ago). Ammonites died off at roughly the same time as flightless dinosaurs. Ammonites were a unique group of creatures, likely having eight separate arms, resembling a coleoid (squids, octopuses, and cuttlefish), while the shell and it's shape closer resembling a nautilus. An estimated 10-20 thousand species of ammonite have been discovered, so no two fossils will be the same. The largest ammonite specimen found was over 1.8 metres (approx. 5.9 feet) in length, while being an incomplete fossil. Ammonite can be found at any location where prehistoric oceans once were. Ammonite is often used as an index fossil, being used to date the approximate age of the rocks it is embedded in. Ammonite is considered to be one of the world's rarest gemstones when the shell appears iridescent.
It is crucial to be aware of laws and regulations governing fossil collection in your area. Many places require all fossils found to be sent to a palaeontologist, and have strict regulations on the selling of locally found specimens.
More information about ammonites can be found here.
Purpurite is a violet to deep red manganese phosphate mineral that contains various amounts of iron depending on the location mined. Purpurite is the lithium-deficient oxidation material produced by lithiophilite. Purpurite appears almost metallic, with an iridescence when hit with light at the right angle. The mineral is named after the latin word for purple, purpura. Purpurite is a relatively rare stone, discovered in 1905 in North Carolina, USA. Purpurite mines can be found in the USA, France, Namibia, Portugal, Rwanda, Malaysia, and Australia. This stone has a hardness of about 4.0-4.5 on the Mohs scale, making it relatively soft and not a common material for faceting, while still being used for jewelry on occasion.
Pyrite is the most common sulfide mineral worldwide, consisting primarily of iron and sulfur. Pyrite forms in unique patterns, which change drastically depending on the environment it forms in, ranging from stalactitic to globular and many forms in between. These minerals can be found globally, with notable deposits being in the United States and several locations around the Iberian Peninsula in western Europe. Pyrite is often referred to as "fool's gold", considering that it is frequently mistaken for gold by the untrained eye.
Pyrite vs. Gold:
A major differentiating factor between gold (shown below) and pyrite (shown at top) is hardness and malleability. Pyrite has a hardness of 6.0-6.5 on the Mohs scale, making it relatively difficult to scratch. Gold has a hardness of only 2.5, making it extremely easy to mold, scratch, and bend. Pyrite is occasionally accompanied by small amounts of gold, however not concentrated enough to be considered a reliable source material. Gold and pyrite also differ between their formation structures, caused by reactions with their respective elements. Gold forms in flakes or nuggets, and are usually found in small quantities, while pyrite forms in considerably larger amounts and in more complex angular shapes.
Lapis lazuli is a semi-precious metamorphic stone, sought after for its intense blue colour. Lapis lazuli, more simply known as lapis, consists of pyrite (see post about pyrite here), calcite*, and lazurite*. (lazurite is what gives lapis that bright blue colour seen above.) Lapis lazuli can be found in mines across the globe, including: Afghanistan, Chile, Russia, Canada, Angola, Argentina, Pakistan, Italy, India, the USA, and Myanmar (prev. Berma). Lapis lazuli has been used practically since as early as 7000 BCE for worship purposes. Since then, the mineral has been used to make the ultramarine pigments used by Renaissance and Baroque artists, as well as jewelry, decorative pieces, and has notably been found in the funeral mask of Egyptian pharaoh Tutankhamun. Lapis has a hardness of 5.0-5.5 on the Mohs hardness scale. Lapis lazuli gemstones can be sold for less than $1 USD per carat for low grade pieces, and up to $150 USD per carat for superfine pieces.
*Detailed information about this mineral coming soon.
Childrenite is a hydrated phosphate mineral consisting of iron, manganese, aluminum, phosphorus, oxygen, and hydrogen. It is usually translucent in shades of yellow or brown, but can also be colourless. It can be found in granite pegmatites and hydrothermal vein deposits in Brazil, the UK, the USA, and Germany. Childrenite has a pyramidal formation structure, and is considered a semi-precious gemstone. Childrenite normally retails for about $30-$70 USD per carat, making it one of the most affordable gemstones.
Galena is a natural lead ore and a common source of silver. It is one of the most abundant sulfide minerals. Galena forms in cubic patterns, creating a blocky texture. Because lead is an extremely dense material, galena is particularly heavy. It has a hardness of 2.5 on the Mohs scale, making it rather fragile. Galena is used not only as a lead source, but also as a semiconductor and cosmetic ingredient. Alternate names for galena include: Lead Glance.
Rose quartz is a common pink to rose red variety of quartz. It is considered a semi-precious gemstone. Rose quartz has been used for beads and jewelry since about 7000 BCE. It is about a 7.0 on the Mohs hardness scale, making it a durable and versatile stone. Rose quartz is considered a symbol of love and healing.
Cerussite is a lead-based mineral formed by carbonated water on the mineral galena (see post about galena here). It can be found in Namibia, Spain, the USA, and Australia. Cerussite is a secondary source of lead, and can be used (less frequently nowadays after the discovery of negative long term side effects to lead exposure) as a key ingredient in paints, bullets, batteries, cosmetic products, and plumbing. It is often white, yellowish, or colourless, but can also occasionally appear in grey, blue, or green. Cerussite uniquely reflects light in a phenomenon called dispersion, creating kaleidoscope-like effects. It also creates a fluorescent yellow hue when exposed to ultraviolet light, with its vibrancy relying on its heavy lead content. Cerussite should be handled carefully and minimally, with personal protective equipment, to reduce risk of lead-based illnesses and symptoms. Alternate names for cerussite are: White Lead.
