Al Stahler: Stories in the rocks
You and I have stories – stories about things that have happened to us, stories about how we’ve responded to what’s happened. Our stories are shaped by our response to our environment.
Rocks – and the minerals of which rocks are made – these, too, respond to their environment. Rocks and minerals, too, have stories.
Some minerals are so special – perhaps because of their color, or their clarity, or their sparkle – we consider them gemstones.
You and I, rocks and minerals, gemstones and potter’s clay, are built of atoms – atoms glued together, in specific combinations, in specific shapes.
The “glue” that holds atoms together, one to another, is electromagnetism – the same force that holds a magnet to the door of the fridge, the same force that makes a bunch of electrons jump through the air in a spark, when you take off a sweater (making the air glow as they bash their way through).
Electrons are subatomic particles – tiny particles, of which atoms are made. Electrons push each other away, but are pulled in by other parts of the atom. Once inside the atom, electrons arrange themselves in patterns.
The attraction of two atoms for the same electron glues atoms together. The pushes and pulls among atoms and electrons … along with the patterns electrons make in their atoms … mold clumps of atoms into shapes … the shapes of the proteins in our bodies, the shapes of the crystals of gemstones.
The shapes of our proteins, and the atoms they comprise, bear witness to the evolution of our bodies: Over billions of years, various combinations of atoms, in various shapes, have proved better than others at digesting food, contracting muscle, fighting a virus.
The atoms of our bodies carry stories, stories biologists like to read. Like all stories, the stories told by the atoms in our bodies hark back to the experiences of the story-tellers – of the environments in which our proteins evolved.
Minerals – gemstones – also have stories to tell … stories that hark back to the experiences of the story-tellers, the environments in which they grew. These environments differ in regards to heat and pressure, moisture, and oxygen.
As story-tellers, rubies and sapphires have very similar tales to tell … both contain two atoms of aluminum, for every three atoms of oxygen. But a crystal composed of nothing but aluminum and oxygen is colorless. The colors of the gems mark where their stories diverge.
Rubies crystallize in an environment that includes, not just atoms of aluminum and oxygen, but also atoms of chromium (the metal that once gave the shine to car bumpers). A few – a very few – chromium atoms made their way into the crystallizing ruby.
Electrons in a crystal push and pull, gluing atoms together, giving the crystal its shape.
Electrons also absorb light … light of very specific colors. The chromium atoms in ruby absorb several colors. What they do not absorb – what they allow to shine through – is red.
Sapphires, on the other hand, crystallize in an environment that includes atoms of iron and titanium. A few – a very few – atoms of iron and titanium link into the crystal of sapphire. Like atoms of chromium, atoms of iron and titanium also absorb colors … but iron and titanium allow, not red, but blue to shine through.
Rocks and minerals have many stories to tell – not just stories of how they formed, but also what happened to them over the millions or billions of years after. The serpentine that is common in the foothills – serpentine that weathers to soil rich in heavy metals – was once rock deep beneath the sea. Plate tectonics (famous for separating Africa and South America) slammed the ancient seafloor into North America. Some seafloor rock squeezed upward … pressure and heat and moisture changed – metamorphosed – deep-sea rock into serpentine, which stuck to the edge of the continent.
A six-wheeled robotic geologist landed on Mars last week, to begin searching for stories told by Mars rocks.
The word “organic” – today – refers to food grown without pesticides. But among chemists, “organic” has an older meaning: organic chemicals are those built with carbon atoms (thus, to a chemist, DDT – with lots of carbon atoms – is an “organic” chemical). When the word was first coined, several hundred years ago, it was thought that all molecules built with carbon were made by living organisms … thus, “organic.”
The rover Perseverance – “Percy” – will search for organic molecules in the rocks of Mars … searching for evidence that Mars – billions of years in its past – once hosted organisms.
Finding evidence of past life on Mars would imply that life is likely to exist on other planets, circling other stars. It would also add a clue to the thought that – maybe – life evolved first, not on Earth, but on Mars … and then migrated to Earth … which would make each of us descendants of (single-celled) Martians.
That’s another story.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at firstname.lastname@example.org.
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