colour on band

hey there. i inherited this interesting ring from a relative who passed away about two years ago. she did not know where the ring came from but stated she found it.

a 10K is stamped inside the band. i am curious about the black discolouration within the design. i tried googling but didn’t find a great answer.

is this cleanable or fixable? or is this just how it looks forever now? what is going on here?

a gentle brush (dry, unused toothbrush) does nothing so i know it isn’t easily removable dirt.

yes i know the one opal is cracked. i love it and will wear it anyway

TIA!