Here is a beautiful coin for many reasons, and yes it is rare too!  It is a solidus of Tiberius III from Sardinia.  First, the Sardinia mint was only in production from 690 to 740 during the reigns of Justinian II (First Reign) through Leo III.  The output appears to have been limited, as there are very seldom pieces offered for sale in the marketplace.  What makes this coin so special is the condition.  It would not be too hard to say that this is the finest piece known not only for the type and the emperor but also for the entire mints production.  The fields are polished and the engraving is crisp.  The coin is not perfectly centered, but has considerably more legends than normal (more than three quarters are full and legible). (Normally, if a quarter of the legends are present you consider the piece to be worth purchasing.)  The coin is also struck on a smaller flan almost like those of Carthage during the reigns of Heraclius through Constantine IV.  The piece weighs 3.77g (which is normal for a solidus from Sardinia) and exhibits a die crack/failure on the right side of steps 2 and 3 on the reverse. The Sear number for this coin is 1372. If there were just more coins that looked like this!