The UK assay offices will be able to answer your questions in detail
Except for very small items it is a criminal offence to sell and item
as gold or silver if it has not been hallmarked.
You will need to buy a registered punch from them to identify
yourself as the maker and then send in each piece, together with it’s
chain or other bits, for hallmarking before you enamel it.
They will charge you for each item.
In striking the hallmark they often mar the work, sometimes using
such force as to bend or distort the piece. Often the mark comes
through to the other side of the piece as a bump in the metal. In
order to avoid this you may need to ask them to mark in a specific
place or warn them about the fragility of the item. They enjoy working
with craftspeople and are usually very helpful.
You will need to think about the hallmark as you design!
Gold and Stone