I didn't really understand until recently where these nodules came from. People in China are selling bunches of them, and they are very beautiful. I thought they must be electrowinning nodules (built up by electroplating), considering how similar they look to the nickel and copper electrowinning nodules I have.
Fortunately reader Eric Winter has kindly pointed me to this article which indicates they are probably vapor deposited in the "Pidgeon process".
Source: eBay seller artgarden2005
Contributor: Theodore Gray
Acquired: 16 December, 2006
Text Updated: 9 April, 2009