Ole is right. Aqua regia is about the only solution that will dissolve gold and platinum. It is composed of equal parts concentrated nitric acid and hydrochloric acid. You will probably find it easier to purchase the gold chloride than the other two nasties.

My experience with this technique is limited, but I remember a lot of fuming, heat release and the obvious requirement to work under a very specialized chemical hood. The reactions can get out of control thermally with the release of clouds of brown nitrous(ic) oxide fumes - not a pretty sight. Once dissolved, the whole mass must be neutralized with lots of caustic soda, ya da, ya da, ya da. Not an easy project and not for amateurs.

Thirdly, in these times of home-made you-know-what, the procurement of concentrated nitric acid will no doubt be an issue as it can easily be use to make you-know-whats. Lets just say that I'm glad my experience is in the far distant past in a galaxy far, far away. That is why my companions changed my nickname from "rocket bob".