Looks like "tinned" copper.
The marks look like someone's drivers license number.
Information in section 7 with enlargeable photos above section.... http://www.ibiblio.org/hyperwar/NHC/books/Navy_Marks/navy_marks.htm
Looks more like YS than SI.
Have you found any platinum pieces with your magnet?
It's a cannon/artillery piece,but the U.S. navy has had crossed cannons for probably around 100 years"Gunners Mate".
A photo of the whole shirt could help.
Looks like 3 corners are chipped and something strange on the left side like a crack and the metal holding the stone seems to have a piece...
In the top photo,is the bottom right corner broken or just the way the photo came out?
I prefer tung oil easy to use and reapply when necessary,not sure what you have in your country..........
One of these? https://www.mullencoins.com/blogs/entry/the-rare-new-2019-w-america-the-beautiful-quarter-dollars
Looks like onyx.
For many years facet designs were a "trade"secret by private cutters and companies. Actual cutting diagrams were published probably in the 1940's...
Looks more like a Satyr ..... https://www.saatchiart.com/art/Painting-Satyr/903336/3939343/view
#5 unakite,sodalite,quartz,jasper and a few leaverite.
Easy to tell,just shine a light through the bottom of the stone and photo from the top.
The stone is rectangular with a step cut pavilion and scissors cut crown,not really unusual.
The steel was cut out for the copper piece.The top edge of the steel was under cut,the copper plate placed into the recess.The 2 end areas and...
The metal and stone would need to be tested.The stone could be an emerald,but can't tell from the photo if it is natural or man made.
The stone appears to be tourmaline. 585 is 14K gold,the amount of copper in the alloy changes the color.More copper makes it redder and more...
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