Historical Rules · 1 February 2013
Rule 206.2
Official source text from the 1 February 2013 Rules release
The color of the expansion symbol indicates the rarity of the card within its set. A red-orange symbol indicates the card is mythic rare. A gold symbol indicates the card is rare. A silver symbol indicates the card is uncommon. A black or white symbol indicates the card is common or is a basic land. A purple symbol signifies a special rarity; to date, only the _Time Spiral_(R) "timeshifted" cards, which were rarer than that set's rare cards, have had purple expansion symbols. (Prior to the _Exodus_(TM) set, all expansion symbols were black, regardless of rarity. Also, prior to the _Sixth Edition_ core set, with the exception of the Simplified Chinese _Fifth Edition_ core set, _Magic_ core sets didn't have expansion symbols at all.)