My Boot-Box of Rocks

5 March 2019

Some people store collectibles in shoe-boxes, but for me I need a boot-box to store my rocks.

Why rocks? For some, it’s the polished beauty of a gemstone that tickles their fancy or the perfect shape of a well-cleaved crystal which strikes awe. However, you will find my collection dominated by earthy colours and imperfect shapes. I find that the greatest thing about a rock is the story it tells.

As a youngster, I would spend a lot of my pocket money on small gemstones at the local health food store. These were small, colourful and polished to catch the eye. I collected them by the dozen. I even bought a copy of The Crystal Bible, only to be disappointed to find more information about chakras than the stones themselves. There’s a common perception that scientific understanding removes the ‘magic’ and romance from an object, but I couldn’t be more in dissent.

My favourite gemstone from those days was the tigers’ eye. It comes in many colours, particularly red and brown, but its distinctive feature is its thin banding. How this banding forms is the most interesting feature of the gem.

Many tiger’s eyes are small pieces of banded iron formations (BIFs), which are massive stone formations kilometres thick and many kilometres wide. BIFs are bands of iron-rich haematite and chert, which individually are only centimetres thick. It is these ancient rocks, up to 2.6 billion years old, that provide much of central Australia’s most impressive landscapes. But these rocks could only begin to form when the first bacterial life began photosynthesising light and oxygenating oceans and skies. This process drove the most dramatic climatic event of our planet’s history, Snowball Earth, when the earth was covered in ice as potent greenhouse gases combusted away. Glacial deposits became the dominant rock in this era and BIFs disappeared deep into the crust.

For the avid rock collector, nothing beats the experience of finding your first fossil! Over two summers, I volunteered with Dinosaur Dreaming, the Cretaceous period fossil hunt run by Melbourne Museum. It’s hard work, breaking large chunks of hard, grey sandstone into pieces the size of sugar cubes in the hot sun and sandy wind of the Cape Otway coast. I kept my first bone, which was an underwhelming coin-sized brown patch, likely from a fish. More exciting was my third find, a slightly squashed, but still whole, vertebra. This bone was kept by the museum for later preparation and research, for it likely came from a small polar dinosaur by the name of Leaellynasaurus.

Leallynasaurus lived some 100 million years ago in the polar forests of Victoria. The climate was far warmer than today and the polar regions were covered in conifer forests. Australia was rifting apart from Antarctica at the time, which produced wide and deep river systems to rival the Amazon. Among these forested valleys lived tiny, agile herbivores like Leaellynsaurus, as well as predatory newt-like amphibians the size of crocodiles, named Koolasuchus. All the clues to this alien world, from the fossils to the ripple marks left by rivers, can be found under the sands along the coast of Cape Otway and Gippsland… if you just know where to look.

The prized items of my collection are the stunning gemstones I collected whilst on a field-trip to Broken Hill. For third-year Geology, one can choose to spend two grueling weeks under the beating sun criss-crossing the desert-like scrub, looking for the next outcrop to map and observe. But on our day off, we visited a local rock collector within whose shed lay a magnificent collection of huge, polished gemstones expertly arranged. He took us out along dirt tracks to find impressive gems of our own. Amongst the gems, my favourites were the many garnet crystals I took home. Their earthy colours may be relatively dull, but it is their habit of forming perfectly shaped dodecahedrons that impresses me. What awesome forces could produce such a crystal?

Broken Hill is on the Eastern edge of the Australian Pre-Cambrian Craton, a landscape dominated by deserts above, containing ancient rocks below. Rocks deposited in shallow, lifeless seas over a billion years ago were then buffeted by continental collisions and crushed by growing mountain ranges. Rocks that befall this fate are called metamorphic, because, much like an insect from a larva, they are reborn; their crystals are regrown under immense heat and pressure. For my little garnets to form required temperatures above 530° Celsius and over four thousand atmospheres worth of pressure. I struggle to comprehend the billion-year ordeal this 50-cent piece-sized gem underwent before being dug up by a curious human.

My years of amateur rock collection and studying our Earth’s natural history have imbued me with a deeper appreciation for both the fragility and power of nature. I hope that by sharing such stories, we can all learn to appreciate the full 4.6-billion-year story of the earth beneath our feet.


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