Garnet is January's traditional birthstone and the stone of Aquarius, and there is nothing to say it is definitely feminine or masculine. In fact, in 2010 there were 5 baby girls named Garnet and 7 baby boys named Garnett (with two t's, so I'm not sure what the parents intended, although this spelling was used as a surname). In 2011 there were 5 baby boys named Garnett, but no Garnet. There were 11 baby girls named Garnet in 2011, and no Garnett. In my mind, Garnett (like Garnette) looks better on girls and Garnet seems more masculine, thus it would look better on boys. However, for the past two years Garnett has been strictly for boys and Garnet strictly for girls. Maybe it's a surname vs gemstone thing. It last ranked on the popularity chart in 1944 at #956, after having ranked for at least a decade. It was most popular in 1904 at #593 for boys, and 1911 at #376 for girls.
Garnet was used along with other gemstone names in the late Victorian era. Garnet is Middle English, an adjective, meaning "of dark red color," a gemstone that is from pinkish red to blood red. It was named after the color and shape of pomegranate seeds, although some say it was named after grain and red dye. As a surname, Garnet could refer to someone who sold pomegranates, as it means "red like a pomegranate." There have been several well known namesakes with Garnet as a given name or Garnet/Garnett as a surname, such as the Victorian soldier Sir Garnet Wolseley, adding his namesake to the list of reasons it was given in the Victorian era. Other namesakes include Henry Highland Garnet, and African-American Abolitionist, and Garnet Clark, and American jazz pianist. Even further back in time there was Garnet IV, King of the Picts.