The keyword term in question is a noun phrase. Its function is to name a specific concept: the characteristic hue of the gemstone assigned to births occurring on the eleventh day of September. The primary birthstone for the month of September is the sapphire, making its quintessential color a deep, rich blue.
The association between a birth date and a gemstone color is determined by the monthly birthstone designation, not by the specific day. For any date within September, the designated gemstone is the sapphire, a variety of the mineral corundum. While sapphires are famed for their blue color, they naturally occur in a full spectrum of hues known as "fancy sapphires," including pink, yellow, purple, and green. The only exception is red corundum, which is classified as a ruby. Despite this variety, the traditional and most widely recognized birthstone color for September remains the classic blue of the sapphire, a color historically symbolizing nobility, truth, and wisdom.
Therefore, for any practical or symbolic application, the color linked to a September 11th birth is blue. This conclusion is based on standardized birthstone charts used by jewelers and gemologists. While an individual could select a fancy-colored sapphire, the primary and historically significant color that answers the query is the range of blues characteristic of the traditional September birthstone.
