Color Notes for Emeralds from Colombia and Green Icebergs
" The color of emeralds is a fascinating story of geology, chemistry and the mighty influence of tiny amounts of impurities like chromium, vanacium and iron. According to George Bosshart, the Colombian emerald is blessed with a near-perfect color chemistry: varying concentrations of chromium and vanadium, with very little or no iron. He adds that the dynamic, violent process of geological formation leads to varying concentrations of these trace elements, even within the same mining region. This makes it hard to generalize about the perfect chemistry, except to say that it is the very low (or absent) iron concentrations, rather than the ratio of chromium or vanadium, that make Colombian emeralds ideally colored.
The color discussion can be continued by an amazing 2022 article from National Geographic called 'Icebergs Can Be Emerald Green.'
Icebergs are most often white-blue objects. But nature is also capable of creating startlingly green icebergs, and they can be found only in Antarctica. Although the scientific literature is full of reports of these emerald ice blocks going back more than a century, no one could explain where they were coming from. Now, a team of researchers may have finally cracked the case.
According to their work, the unusual hue seems to be a combination of two distinct processes. First, bubble-free icebergs need to form at the bases of ice shelves jutting out into the Southern Ocean. At the same time, ground-up yellow-red glacial dust from Antarctica’s bedrock has to be brought along for the ride.
“It’s pretty much Antarctica’s version of mixing blue and yellow paint together to get green,” says James Lea, a glaciologist at the University of Liverpool who was not involved with the work.
"Many U.S. journalists like to refer to these curiosities as emerald icebergs," says U. of Washington professor Steve Warren, "perhaps recalling the 1798 epic The Rime of the Ancient Mariner, in which poet Samuel Taylor Coleridge describes an Antarctic sailor seeing “ice, mast high … as green as emerald.”
Glacial ice tends to have a blueish hue because it absorbs the redder, longer wavelengths of light, leaving the bluer, shorter wavelengths free to be scattered back into our eyes but in parts of Antarctica, the ice is so compressed that it lacks any bubbles whatsoever. This creates a longer pathway through the ice so that, to our eyes, it looks incredibly clear and vibrantly blue.
But it is Iron that create the green color. Iron oxide dust gets eroded away as ice flows over East Antarctica’s bedrock. The resulting powdery sediment, so-called glacial flour, eventually makes its way into the sea, where the rusty substance freezes to the base of ice shelves that brake off and form icebergs. As it happens, iron oxides tend to be yellowy-red. “Ice filters out the red light, and iron oxides filter out the blue light, so green is what's left to escape as refracted sunlight re-emerges from the iceberg,” Professor Warren explained."
