It could well be silver gilt or vermeil as the French call it, and that would explain the patch you mentioned. It's always very difficult to tell from photos.
If that is the case, then the gilding is recent, in other words post-1850 when electro-plating became the norm for applying a coat of gold to silver. There is an older process for making vermeil, called fire gilding using a mercury flux, but this, too, would have been a process unknown to a medieval maker. The only gold plating process known in medieval times was "close-plating" and this piece is not close plated. That's one composition you can tell from photos alone, the look is so distinctive.
That also explains why it was black, as the oxidation of silver underneath will cause this, but is usually easy to remove, often with just a wipe of a cloth.
An expert on medieval jewelry would be able to figure out the precise composition. If it is of the period that its design indicates (12th - 14th century) it certainly won't be anything like our modern industrial golds and silvers in terms of composition.
And no, I have never seen one identical to yours, but then that's almost always the case with medieval metalwork. No two pieces are alike but there are decorative motifs that are the same and these common denominators are what the specialists use to attribute age and origin.
As for value, if it were to turn out to be silver gilt, it would likely mean that it was of recent origin (within the last 160 years) in other words, made in Victorian times in a medieval revival style. In which case it would have an auction value in the range of £150 - £250.
But such a piece would have silver hallmarks somewhere and their absence would therefore be puzzling.
There's a slight possibility that it could be medieval in date and gilded in Victorian times or earlier this century. But that's a little unlikely and besides, a silver piece from that long ago would be pitted with oxidation and not nearly as smooth, even under the surface of Victorian gilding.
If it were solid gold, and confirmed to be medieval, it would have a value in the two to three thousand pounds range.
I wish I could be more conclusive than this, but I'm sure you'll get some firm answers from the right specialist at the Victoria & Albert Museum.
Hope this helps and thanks for asking me about it, it's a really interesting and beautiful piece.