Reference books is a first stop to learn about a coin's rarity, but something further research is necessary. If you want to do some investigative work, these factors need all be taken into consideration.
This is the number of coins produced by the mint, usually in one year. Sometimes dies dated in one year were used the next year and so called overdates exist (often rare) in some years when the new date has been engraved on a die from the previous year, thus leaving rests from the “old” date.
Hoarding is the accumulation of something for preservation, thus limiting supply of this item. In a numismatic context hoarding is the process of coins disappearing from circulation.
A common type of hoarding occurred when the metal value in coinage became higher than the intrinsic value, in which case opportunists would export or melt large quantities of coin for a profit.
Another example of hoarding is small coins – either by size or denomination, or worst both! – that easily get lost or thrown into a jar. Other examples are especially beautiful coins or commemorative coins that would have been put aside and so are available in good quality in greater numbers.
Read more about the biggest coin hoards found…
Workings of the mint
To understand rarity it is important to know the workings of the mint. If, for example, a large number of silver coin was withdrawn in one year you would expect a large mintage the next year. However this may not be the case. The mint could store the majority in vaults for a number of years and gradually melt them and mint new.
Read more: the workings of the mint are crucial to understand the Melbourne mint 1872/1 overdate on the gold sovereign…
Read more: the creation of a super-rarity – the story of the 1933-D $20 gold double eagle…
Other economic factors
Essentially the output of coinage is in large an effect of political and economic factors. Wars, revolutions and other events have great impact on the availability of certain coin types. Another example is piracy and plundering – George Anson’s voyage around the world brought back millions of ounces of silver and gold that produced large amount of coin in England mid-18th century, these have the famous mintmark LIMA under the king’s bust.
Read more about George Anson’s Voyage around the world…