Recently I saw that on the surface of one of my 24k ingots, a small brown
dot developed, and it was while the piece was still in original package
(cardboard/plastic). I thought that some dirt may have entered the space
within, and decided to take it out to see if it can be cleaned. I used a dry
soft cloth but not just that it didn't go away, but the other day I saw two
more of the kind (a few months after the first). They look like some
"greasy" dots of about 1 mm in size and are transparent (like they're done
with liquid). What might that be? There were none when the piece was new and
they showed up on just that one piece.