It is definitely not permissible to use golden and silver plates to eat, drink or make wudu. However, women can use them as ornaments. Men are allowed to wear only silver rings. There is no objection to using things made of materials other than gold and silver.

If the thing is not pure gold or silver and if it consists of some other materials, the material which is in majority is taken into consideration. If gold or silver is in majority, that plate or cup cannot be used. Accordingly, it is obvious that it is permissible to use plates or cups that were dipped in aqua regia because the amount of gold or silver in those plates is less than the other materials.