Gold coated in Black Stone

I have found several nuggets with a black stone attached and in some cases it's like a jet black coating of varying thickness.
Does anybody know what this stone or mineral is ?.
It seems to be closely associated with nuggets in Victoria and I have found it to be of low specific gravity and non magnetic.
Even that nugget Choppacalamari posted looks like is has the same material incorporated.
I haven't tried to clean it off on mine as the contrast between black and gold looks nice as a specimen.
Cheers mark

Most likely hard packed iron coated dirt
Soaked in Alibrite for a day or so and it turns to the familiar rich orange/brown soil colour. Then popped into an ultrasonic cleaner and it comes off in a cloud

MS wrote:I have found several nuggets with a black stone attached and in some cases it's like a jet black coating of varying thickness.
Does anybody know what this stone or mineral is ?.
It seems to be closely associated with nuggets in Victoria and I have found it to be of low specific gravity and non magnetic.
Even that nugget Choppacalamari posted looks like is has the same material incorporated.
I haven't tried to clean it off on mine as the contrast between black and gold looks nice as a specimen.
Cheers mark

Most probably Manganese if it is very dark (jet black). Hydrochloric acid will remove it but be prepared to freshen up the acid every day and also give the nugget a scrub. Usually under the Manganese the nugget will have porous sections so be aware of flour or paint gold coming away in the bottom of the container you acid it in.

JP

Thanks for the reply's
I haven't tried to remove this coating yet and might try a bit of hydrochloric first to see if it touches it, if not I will get some of that albright.
Some of the specimens look good as they are but a couple need a bit of cleaning to show up more of the enclosed gold.
I think your right JP with it being Manganese , I looked up on the net some pic's of manganese nuggets and it does look the same.
I thought manganese was grey for some reason and didn't bother to check it out further.
Hey steve if I use albright do you have to neutralize the Hydrofluoric or is a good wash up in water good enough.
Cheers
Mark

Hi MS,

At first I was just rinsing them after alibright but after a week or so they were discolouring.
So back into the alibright again and then a few laps of the ultrasonic cleaner and no more discolouration even months later. Not sure if that's actually neutralizing the acid or just riding it of the last traces of the stuff which just a rinse didn't do.

G'day MS

After the nugets / specimens are cleaned to your satisfaction, use "Bi-Carb of Soda" and hot water to soak them in to neutralize any left over acid.

cheers dave

Hey mark ive posted this before but id give boss pickling gel ago but make sure u do the bicarb soda thing after your done u can get it as most welding supply places Cheers Glenn

alibright works because it has 1% hydrofluoric acid in it --great stuff for cleaning