Holly, is this a modern brooch? Is it marked 12k? This is not a common control mark. The brooch could be handmade. Not sure. Any chance of an image?
9 carat gold is a common caratage in the UK; its introduction dates from wartime when the UK government allowed assay offices to create this lower caratage. Reason? In order to maintain production of wedding bands which were classified as “essential items” throughout the war itself.
Holly you mentioned a flat disk that looks as if it's been separately made and then attached to the back of the gold mounting of your black onyx oval brooch. It could be handmade. Where there is no compulsory marking, manufacturers/ jewelers usually stamp the jewellery themselves, displaying both their own identifying mark and the caratage or ‘fineness’ of each piece.
