Game concept
Artifact Rarity
The game also links it as Rarity, Common, Masterwork, Famed, Illustrious.
From the game's encyclopedia
Some Artifacts are rarer than others, which is reflected in their Rarity.
The levels are, from lowest to highest:
• Common (never shown since it is the default)
• Masterwork
• Famed
• Illustrious
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Artifact Rarity is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Some Artifacts are rarer than others, which is reflected in their Rarity." It is part of Artifact. The game also links it as Rarity, Common, Masterwork, Famed and 1 more. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.