I have one of 5 known coins of a specific type.
What is the rarity? Is there an official rarity listing? I know there used to be one, but with all of the coins being found.. has it changed?
Rarity is usually defined based on the number of coins in a fixed, clearly defined sample. For example
- number of examples on
Acsearch- number of examples in a specified list of
museum collections - which is the way
rarity is defined in
RIC or in
Sydenham CRR.
- number of examples cited in a reference such as
RPCEach of these samples of course only represents a small proportion of possible number of coins. The purpose is only to give a consistent measurement
standard. So you having one extra coin does not change a
rarity rating that was based on a defined sample, such as a specified list of
collections, or number cited in a specific book. The
rarity remains exactly the same. You just happen to own one of the examples outside those sample databases (which is normal and expected). In fact it's quite usual that when
RPC list for example 5 examples from specified
collections, that there are 30 more out there in the wild.
When you say you have "one of five known", are you sure about that? Maybe there's five on
ACsearch, or five listed in a
standard reference
work. But there might be many more examples out there - including yours. And just because you don't know about other examples, doesn't mean others don't know. There may be more known but just not by you.
So single additional coins do not change a
rarity rating. Even several individual examples don't change a
rarity rating that was based on a specified sample. It is expected - the rating was based on a sample after all. And over time more coins are always discovered but at the same time more coins get locked into permanent
museum collections, or are perhaps lost again. So the gradual addition of coins from new discoveries is often offset by reductions in available coins for collectors for other reasons. On balance, even
rarities defined 150 years ago by
Cohen (who gave prices in Francs as an indication of
rarity) tends to remain unchanged over the long term. In fact the relative pricing of coins in the
Seaby Roman Silver Coins series was initially based on
Cohen 1850s pricing and often those relative pricings have "stuck" and become recognised as an indicator of
rarity and desirability (i.e. if you write down that a coin is
rare, then more collectors will want the
type and they will therefore need to pay more to get it, which makes the
type more difficult to get, which makes it indeed seem rarer!)
What DOES change a
rarity rating is when a massive
hoard arrives.
Roman Republican collectors experienced that a few years back when a large
hoard of "Cassius Tripod"
denarii, all in
mint state, was placed on the market. That coin
type previously
had a
rarity rating of
Sydenham 9 ("exceedingly
rare"). I would now rate it as 4 (merely "
scarce"). But in that case there were hundreds of additional examples of a
type previously known in only a handful of examples. The number of examples went up by a factor of 50 or so, overnight. That's quite different from the normal dribble of occasional extra examples such as you mention which do not change the
rarity rating.
The link provided by
Craig is indeed useful:
https://www.forumancientcoins.com/board/index.php?topic=116436.0