Hi,
I hope this is the right forum.
I've been collecting sea coal for several years and have about 50 pounds in an old bin.
These pieces wash up individually, likely from old wrecks. One wreck a British steamer had picked up a load of coal from Algiers before it ran aground and broke up nearby over 100 years ago.
In my collection are dozens of pieces of peacock coal - all colors ranging from gold to green to pink and purple.
There isn't much written on peacock coal but the general description - including here on mindat- is a "thin film" on the piece.
I feel that's incorrect since the peacock coal I find has been in the sea, tumbled smooth for decades, yet the colors are as brilliant as ever. Wouldn't a "thin film" be worn off?
I also have a new observation. I mistakenly left a piece of peacock seacoal in my pocket and ran it thru the washer & dryer. The outside color stayed. But the piece also broke open revealing pockets of these colors INSIDE the coal nugget.
I took some photos and will try again in sunlight when we get some but I wanted to share these. They clearly show the pockets of color inside the broken piece.
Is my thinking right? Please let me know what you think. Thank you.
I'm no expert on coal, but yes, these colors wold be due to a "thin film" phenomenon. What that thin film is, good question. For one, coal is not pure carbon. It is mostly carbon, but also contains varying amounts of carbon-based chemicals, as well as some bit of layered silt- and clay-sized sediments. The colors seen depend on the thickness of the layers of....whatever; but, they are thin, about the wavelength of the color of the light. One layer may get eroded off, but then another may appear because of that. Probably not the best explanation of what you are seeing.... Seeing them on the inside just means the piece of coal had internal fractures, as would be expected for a brittle material bashed around by the waves. I'm very curious what the coating is made of - Some hydrocarbon that partially oxidized or hydrated? But we'll probably never know, because analyses of such extremely thin films is very very difficult. Doug Daniels Wrote:
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> I'm no expert on coal, but yes, these colors wold
> be due to a "thin film" phenomenon. What that
> thin film is, good question. For one, coal is not
> pure carbon. It is mostly carbon, but also
> contains varying amounts of carbon-based
> chemicals, as well as some bit of layered silt-
> and clay-sized sediments. The colors seen depend
> on the thickness of the layers of....whatever;
> but, they are thin, about the wavelength of the
> color of the light. One layer may get eroded off,
> but then another may appear because of that.
> Probably not the best explanation of what you are
> seeing....
Thanks Doug, I've been trying to understand this for a long time. I read that miners used to ignore the coal bc the colors would "wear off"... not my experience.
And Mindat has the definition as the colors on a film on the "surface" of the coal... which this piece shows the color inside.
I'd love to know the source of the color. It's absolutely beautiful.
Alfredo Petrov Wrote:
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> Seeing them on the inside just means the piece of
> coal had internal fractures, as would be expected
> for a brittle material bashed around by the waves.
> I'm very curious what the coating is made of -
> Some hydrocarbon that partially oxidized or
> hydrated? But we'll probably never know, because
> analyses of such extremely thin films is very very
> difficult.
Thanks Petrov,
All the colors got inside one of those cracks. If you zoom in you can see a line in the blue color but in real life that's a rainbow of every color inside this piece. I'm amazed by it. I wish I could capture it with my photo.
Do we know what makes antracite coal blue? Many of the pieces I find are blue only but then some are crazy rainbows.
Could a reaction with seawater be causing it? On the beach sometimes is foam with bubbles colored just like this peacock seacoal.
I am amazed we don't know what causes it.
Thank you. Are you sure you have anthracite? And, no, it isn't seawater that makes the color (the thickness of the foam causes the color.....). Again, for the coal, one of the mysteries that we may never know the answer to. 
I suspect the color is caused by the same phenomenon - and perhaps maybe even very similar chemicals - as causes a 'rainbow' on a puddle.
A puddle rainbow is caused by a very thin layer of hydrocarbons and related materials from gasoline, grease, etc. These materials collect on the surface of the puddle because they don't dissolve in water. Coal also contains similar materials, which cannot dissolve in the solid coal. So these materials collect in what ever minute spaces are available - seams, cracks etc. When the coal is broken open their presence is revealed.
