Backing away from rarities for a moment, I want to show a piece of historical importance in regards to the changes to the coinage in the later part of the Byzantine Empire.  This piece is a Tetarteron Nomisma of Nicephorus II, Phocas (963-969).  It is accepted that during his reign (or possibly earlier during the short reign of Romanus II), there was a currency reform that created two gold pieces, the Histamenon and Tetarteron.  The Histamenon was to be of the same weight and purity as the Solidus of previous reigns.  The Tetarteron was the equivalent of a 22 Siliquae piece of the Sixth and Seventh centuries, but most likely was produced to facilitate trade in the East where the Fatimid dinar standard was being used.  These two denominations continued throughout the next two hundred years, with many changes to size, shape and purity levels.