Elementary: Sherlock’s Lapel Pin(s)
Taken from hi-res promo stills:
And then there’s this one (also from a promo still):
Which may be the same pin, desaturated or in silver. Or not. The three above appear to be the Arethusa coin / pin / replica.
There are at least two other pins, however: The much-rumored Royal Marines pin and what appears to be a gold Sovereign:
Gold (?) full Sovereign coin / replica / pin on the left, Royal Marines pin on the right. The above images have been taken from promo stills / screencaps. Below are examples of the ‘real’ found objects, as seen on the internet:
Again, because I do not know their exact size, I cannot be sure what they are (pins, cufflinks, tie tacks, replicas, etc).