This is a coin type I didn't even know existed before purchasing this example. It is a tiny AR quinarius of Caracalla (198-217 AD), struck in 213 AD. Along with all the denarii and antoniniani struck during his reign, Caracalla also minted a very small number of quinarii.
This coin is listed as "Rarity 3" in RIC and all the examples I have found carry hefty price tags. This is a decent example, with good details on both sides, struck on a nice silver planchet. Many of the examples online are corroded or have surface issues.
I purchased this coin on
ebay in uncleaned condition. The seller had it properly attributed but in its uncleaned condition it didn't attract much bidding interest. It cleaned up nicely using lemon juice, distilled water, a soft nylon brush and a couple of Q-Tips.
(Diameter: 15 mm. Weight: 1.24 grams - RIC, Vol. IV, Part 1, #317a)
Here is the seller's photo:

This is the photo I took after receiving the coin:

Here is the finished coin:
