die Seltenheit
rarity
The state or quality of being rare or occurring only in small numbers.
Die Seltenheit dieser Briefmarke macht sie für Sammler extrem wertvoll.
The rarity of this stamp makes it extremely valuable to collectors.

rarity
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rarity
The state or quality of being rare or occurring only in small numbers.
Die Seltenheit dieser Briefmarke macht sie für Sammler extrem wertvoll.
The rarity of this stamp makes it extremely valuable to collectors.

'Die Seltenheit' (rarity) is a noun derived from the adjective 'selten' (rare). The ending '-heit' is often used in German to form an abstract noun from an adjective that describes a quality or state.
Die Schönheit der Landschaft war atemberaubend.
The beauty of the landscape was breathtaking.
You can easily distinguish the two meanings: when 'Seltenheit' (rarity) refers to the abstract quality, it is uncountable. When it describes a concrete rare thing or person, it is countable and can appear with an article like 'eine' (a) or in the plural.
Die Seltenheit des Materials treibt den Preis in die Höhe.
The rarity of the material drives up the price.
When 'Seltenheit' (rarity) describes the abstract quality, it is often followed by a genitive or the construction 'von + dative' (of + dative) to specify what is rare.
Die Seltenheit seltener Erden ist ein globales Problem.
The rarity of rare earths is a global problem.
exception
'die Ausnahme' and 'die Seltenheit' can both describe that something does not happen often. The difference lies in the perspective. 'Ausnahme' is used when there is a rule, an expectation, or a normal case, and one case deviates from it. 'Seltenheit' only emphasizes that something is rare or occurs only a few times. Learners may confuse the words because exceptions are often rare. But not every rarity is an exception: a rare animal is a 'Seltenheit', but not necessarily a violation of a rule.

Frischer Schnee im April ist in dieser Stadt eine Seltenheit.
Fresh snow in April is a rarity in this city.
Normalerweise ist das Büro um acht Uhr offen; der geschlossene Eingang war heute eine Ausnahme.
Normally the office is open at eight o'clock; the closed entrance was an exception today.