While usinig the noun hyacinth as an adjective is strange it is even more strange to use the word to modify girl as the hyacinth flower is classically a male symbol.
The Exploring The Waste Land site has a poetic translation of Ovid's Hyacinthus transform'd into a Flower from Metamorphoses, Book . It was
Translated into English verse under the direction of Sir Samuel Garth by John Dryden, Alexander Pope, Joseph Addison, William Congreve and other eminent hands.
Ralph Waldo Emerson in his elegaric poem Threnody mourns his dead son and refers to him as the hyacinthine boy. There are additional reasons to read Threnody as part of the study of The Waste Land.