All the glitters is not gold

I found this stone while out walking yesterday. It has now been added to the pond. I have no idea what the gold like colour is on the stone so if anyone can give me any info I would be grateful. It is not Fools gold.

I watched blackbirds, starlings and sparrows having a bath in the pond this morning, very entertaining

It was a wet cold day today so we did not go out and now we have the fire on.