“Cristales cueva de Naica” by Alexander Van Driessche. This picture shows the Naica Mine in Chihuahua, Mexico, which is home to some of the largest selenite crystals (a variety of gypsum) in the world.
Licensed under CC BY 3.0 via Wikimedia Commons
Welcome to the second instalment of this two-part blog series on the six most awesome rock minerals (for various reasons and in no particular order.) In the first instalment, Part 1, we looked at iron pyrite for its wonderfully geometric crystals and diamond for its many different traits, not least of all its hardness and beauty. Lastly, the limelight was cast on fluorspar for its property of thermoluminescence, which is science speak for “going disco when thrown into a camp fire.”
We have three most awesome minerals yet to examine, but before I get cracking, I need to state that this selection doesn’t even scratch the surface of the sheer diversity of rock minerals, crystals and gems that are forged within the hot and pressurized interior of our planet. There are really so many rock minerals that are awesome:
- Mica forms incredible flat sheets of translucent monoclinic crystals.
- Amethyst derives its name from its ancient medicinal use as protection against poisoning and drunkenness (look how that turned out for the ancient Romans).
- Calcite is special because it double refracts light and its crystals are perfectly-shaped 3D parallelograms.
- Halite is special because it actually tastes like salt – it’s made from sodium chloride – and, if left undisturbed for many, many years, can form giant columns of glittering crystals, as we saw in that picture of the Chandelier Ballroom in Part 1.
- Corundum is awesome because it’s the second hardest substance on the planet, but contrary to its ‘tough as nails’ character, is formed in cute little pink hexagonal tubes. Like miniature pool noodles.
Then, there are all those minerals and elements we covet as rare, beautiful and valuable. My choice has been restricted to those that – while commonly found, as many of them are – are still very special and frequently overlooked. The ones I have selected here are but a mere sampling, which has been done subjectively. Why? Because science. Oh and also this is my blog and I’m the boss.
So… with that administration out the way, let’s don our hard hats, grab our picks and get excavating!
Awesome Rock Mineral # 4: Obsidian
Chemical Composition: Silicon, magnesium, iron and oxygen
Why it makes this list: Its formation process is cool
Name Origin: “Obsius” after the Roman who apparently discovered this rock in Ethiopia.
Star Sign: Haha, just kidding!
Obsidian is a jet black stone with a vitreous (glassy) lustre. Just like glass, obsidian tends to shatter into sharp fragments when hit hard, although it is much stronger than the glass your beer bottle is made of. So, smashing a block of obsidian against your head wouldn’t be advisable, unless you’re the kind of person who would actually smash a beer bottle against your head, in which case knock yourself out.
I call that natural selection.
Obsidian’s strength and brittleness have resulted in its use as sharp cutting implements and weapons, such as spear and arrowheads, some of which date back as much as six million years. Ancient Egyptians found obsidian to offer a suitable artistic representation of the iris. As such, they would use it together with a variety of other coloured gemstones to recreate their dead or dying* pharaoh’s countenance on the front of their solid gold sarcophagi.
* Pharaohs spent more time, resources and effort planning their death than they did enjoying life. They believed that one’s mortal life was but mere preparation for the afterlife. Millions of years later – post science and technology – the majority of the world’s population still believes exactly the same thing.
King Tutankhamen was a tenderly young Egyptian pharaoh (he was 9 or 10 when he became king) who ruled during the 18th dynasty (1332 BC – 1323 BC). This mask was used to cover his mummified remains and contains inlays of, amongst other gemstones, serpentine, lapis lazuli, malachite, garnet and obsidian.
Uses aside, what I find to be most special about obsidian is the way it is formed and it is here that we encounter a very interesting geological pearl of wisdom. The longer magma or molten rock is allowed to cool for, the larger the crystal size of the resultant igneous rock. Makes sense doesn’t it? On the one end of the spectrum, we have granite, which is formed from the ultra slow cooling of magma over many millions of years. The next time you’re bonking your partner on the kitchen counter, take a brief look at the size of the crystals within its polished surface. Big, huh? Well, incidentally, so is the size of the crystals.
In this picture, we can quite easily discern between the three composite rock minerals that make up granite. The pink crystals are feldspar, the white are quartzite and the black is mica.
At the other end of the spectrum, magma that is shock-cooled, in other words cooled really quickly, doesn’t have any time to form crystals and the resultant rock is an amorphous lump of dark brittle glass. So, essentially, what you have just learned is that coarse granite is composed of exactly the same material as glassy obsidian. Yet they look completely different! It’s like Kim and Khloe Kardashian!
So… how can you shock-cool magma? The usual method employed by Mother Nature is ejecting it at a few hundred kilometres an hour out of an erupting volcano, at which stage it theoretically becomes known as lava. The molten rock cools from approximately 1000°C (1800°F) to a little over ambient air temperature in a matter of minutes. The result is obsidian.
The truth is, obsidian is not strictly speaking a rock mineral, just as granite cannot be considered a rock mineral. Remember our Spice Girl analogy in part 1? Well obsidian is a complex blend of all the rock minerals that make up granite (feldspar, quartz and mica). As such, obsidian is more correctly termed a “mineraloid.” If I was submitting this blog to my geology lecturer for marks, I would be penalized for lumping obsidian in the same category as iron pyrite, which is a true mineral.
Awesome Rock Mineral # 5: Opal
Chemical Composition: Silicon, oxygen and water.
Why it makes this list: Cos it’s so damn beautiful.
Name Origin: From the Latin word opalus: “to see a change of colour”
If I was a Neanderthal (my mother will argue that I am) and you placed an uncut diamond and a stone of opal in front of me and asked me to choose one based solely upon its aesthetic appeal, I would point at the opal and say: “ug.”
You may snigger at my seemingly ignorant selection, but in addition to its superior aesthetics, high quality opal fetches as much as $20,000 a carat. This, my friends, beats the Chuck Norris of gem stones by a fair margin.
If you have ever closely scrutinized a piece of opal, you will know just how special it is and how very hard it is to explain its unique brand of beauty. Opal is composed of tiny spheres of silica (sand, essentially) which are packed into tight water-bound layers. Water does all sorts of strange things to light. Combine that with the near-translucent silica spheres and the incoming light gets so damn confused that is splits into all seven of its personalities. These bounce back and forth between the layers and eventually exit the stone to be perceived by our eyes. The larger the size of the silica spheres, the more colours we see, while smaller silica spheres tend to refract darker blues and violet.
I could bumble on about opal, but the truth is, this amorphous gem stone is just so pretty, only a picture could do it true justice:
“62cts Brazilian Crystal Opal” by Daniel Mekis. Licensed under CC BY-SA 3.0 via Wikimedia Commons
Is it a kaleidoscope? Is it a laser light show? Am I on acid? I couldn’t say; are you? It’s opal!
Awesome Rock Mineral # 6: Magnetite
Chemical Composition: Iron and oxygen
Why it makes this list: It’s bipolar.
Name Origin: From the name of a Greek shepherd, Magnes, who discovered magnetite on Mount Ida when he noticed his metal-tipped staff sticking stubbornly to the ground under his feet.
We tend to think of magnets as man-made things, when in fact nature is simply bursting at the seams with examples of bi-polar oddities. Magnetite, as its name suggests, is a black metallic rock mineral composed predominantly of iron and it is the most magnetic of all the naturally occurring rock minerals on our planet. Geologists frequently keep a lump of magnetite on their desks as a paper clip dispenser.
Magnetite does, of course, have greater claims to fame: its various properties provide scientists with an insight into fancy-sounding things such as plate tectonics, paleomagnetism and magnetohydrodynamics. I have chosen magnetite for this list because it blows my mind that a seemingly unremarkable rock dug up from the ground can make metal move of its own accord. Of course, it’s not really moving of its own accord, but everyone fantasizes about having telekinetic powers every now and then, even if the object you’re manipulating is a paper clip.
Magneto, eat your heart out!
Class Dismissed: Your Take-Home Message
No, this is not the work of a super talented graphic designer. It’s Chalcopyrite up close and personal.
There’s really only one message I want you to take home from today’s sciencey musings. And that is that even the merest glimpse beneath the surface of any scientific discipline reveals a fathomless volume of absolutely fascinating information about the world around us and, in the context of this article, beneath our feet. Every single gem stone scattered on the floor of your local “Scratch Patch” or “Geology World” is special for many reasons that extend beyond their appearances, just like every single human being is. Unless you’re Paris Hilton.
Now, THAT’S hot.
Wander Woman Thea 
