Amanda is a retired educator with many years of experience teaching children of all ages and abilities in a wide range of contexts.
The study of rocks is known as geology. All types of rock fall into one of three categories:
Let's look at what each of these means in turn.
Igneous rock starts off deep within the earth as magma (molten rock). The magma rises toward the surface where it may erupt from a volcano, or cool and solidify within the earth's crust.
Igneous rock that erupts from a volcano and reaches the earth's surface is called "extrusive". Igneous rock that solidifies before it reaches the surface is called "intrusive".
Rocks are weathered into fragments that are carried away by water, wind, and ice. These sediments are laid down in lakes, rivers, sand dunes, and on the sea floor. Over millions of years they are compressed, forming layers of sedimentary rocks.
Metamorphic Rock is igneous or sedimentary rock that has been changed by heat and/or pressure. Heat may come from rising magma, and pressure may occur when rock is squeezed during the formation of mountains.
2. Geological Timescale
Rocks are dated according to a geological timescale that divides the earth's history into eras, periods, and epochs.
|Era||Period||Ma (Million years ago)|
Precambrian (about 7 times longer than all the other periods combined)
4,600 (to the origin of the earth)
3. The Rock Cycle
All rocks are constantly passing through a recycling process.
Igneous rocks are weathered away and washed into the oceans. Mineral particles sink into the sea floor where they are compacted into sedimentary rocks. Heat from molten rock changes surrounding sedimentary and igneous rock into metamorphic rock. Molten rock in the magma may rise to the surface where it cools forming new igneous rocks.
4. Marvelous Minerals
A mineral is a natural, non-living substance. Examples include gold, silver, gypsum, quartz, and sulfur.
Different combinations of minerals form different kinds of rock. Granite, for example, is composed of three minerals: quartz, feldspar, and mica.
Ore minerals contain metals and about 80 types of pure metal are extracted from them. The ones we most commonly see and use in everyday life are:
- Rutile, from which we extract titanium. Titanium is a light, strong, flexible metal used in building aircraft.
- Bauxite, from which we extract aluminium. Aluminium is used in construction and the manufacture of goods such as cans and saucepans.
- Galena, from which we extract lead. lead is the softest of the common metals and we use it in making batteries and in engineering.
- Hermatite, from which we extract iron. Iron is used in construction and as an ingredient in making steel.
- Chalcopyrite, from which we extract copper. Because copper is an excellent conductor it is widely used in electrical applications.
- Cinnabar, from which we extract mercury. Mercury is liquid at room temperature and is used in making scientific and medical instruments and in the manufacture of drugs and pesticides.
5. Incredible Crystals
Crystals grow from molten minerals, or minerals that are dissolved in liquids, such as water. Of the earth's rocks and minerals, 85% are formed from crystals.
All crystals can be categorized according to their geometrical shape, known as its "system", and its physical hardness plotted on a scale of 1 to 10.
Diamond, galena, garnet
Zircon, rutile, vesuvianite
Sulfur, olivine, topaz
Rhondonite, kyanite, turquoise
Measuring Mineral Hardness
The hardness of a mineral is graded on a scale of 1 to 10 devised by the German mineralogist, Freidrich Mohs (1773 to 1839).
Mohs Mineral Hardness Scale
6. Mineral Structure
A mineral's hardness depends on the arrangement of its atoms. Diamonds and graphite are different forms of the same element, carbon, but their hardness varies because of their different internal structures.
Diamonds are the hardest of all minerals. Each atom is strongly bonded to four others, forming a compact, rigid structure.
In graphite, the atoms are arranged in layers that easily slip over each other. This gives graphite its weak structure.
Gemstones are minerals valued by humans for their beauty, rarity, and durability. There exist about 100 different types of gemstones. The most valuable include diamonds, emeralds, rubies, and sapphires.
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Found in Russia, South Africa, Australia, and Brazil
Found in India, Thailand, Burma, and Sri Lanka
Hardness: 7 to 8
Found in Russia, USA, Zambia, and Colombia
Organic gemstones are those which have a plant or animal origin. They include pearls, shell, jet, and amber.
8. Giant-Size Gems
The largest diamond was called the Cullinan. It was found in Africa in 1905. It weighed the same as a full-grown pineapple.
The largest pearl is called the Pearl of Lao-Tze. It was found in the Philippines in 1934 in the shell of a giant clam. It weighs about the same as a four-month-old baby.
9. Carats and Beans
The weight of a gemstone is measured in carats.
1 carat = 0.2 g (0.007 oz)
The term carat comes from the Greek word for carob seed. These little beans were once used as weights.
The purity of gold is also measured in carats. The purest gold is 24 carats.
10. Rocking Out
- More than half all the gold mined returns to earth - as most of it is buried in subterranean bank vaults!
- The word crystal comes from the Greek word kyros, meaning icy cold. It was once thought that rock crystal was ice that had frozen so hard it would never thaw
- About one in every thousand oysters and one in every three thousand mussels, has a pearl inside
A Last Word
I hope you've enjoyed these top 10 interesting and fun facts about rocks, minerals, and crystals. Both geology and mineralogy are fascinating subjects which will reward you well for the time you put into studying them. To be a professional geologist you'll need to do well in geography and chemistry at school and then go on to study for a degree in natural sciences. Good luck!
© 2019 Amanda Littlejohn
Amanda Littlejohn (author) on March 25, 2019:
Thanks, Kathi. I'm so glad you enjoyed it.
Kathi Mirto from Fennville on March 25, 2019:
Great article, lots of good information and photos!
Amanda Littlejohn (author) on January 14, 2019:
A Google image search does turn up pictures of it. The problem was finding one which is legal to use for commercial purposes. It's not a very pretty thing, that pearl, as you'll see if you search for it.
Bill De Giulio from Massachusetts on January 13, 2019:
If you ever do find a picture of it please post it. I had to read that twice, amazing.
Amanda Littlejohn (author) on January 13, 2019:
I'm glad you enjoyed it. I tried so hard to find a picture of the giant pearl, but it seems impossible.
Bill De Giulio from Massachusetts on January 13, 2019:
Another interesting hub, Amanda. Thank you for the geology education. This is definitely a fun topic.