Les Bijoux by Jo Eun-Ha. A Fantasy manga rated Older Teen, published in English by Tokyopop.

In a fantasy world where the ruling Habit class brutally oppresses the Spar commoners who toil in twelve mines, Lapis Lazuli, a being who can switch between male (Lapis) and female (Lazuli) forms, is born to a poor enslaved couple in Neige Mine. When the cruel Habit Diamond brings tragedy to Lapis's family, Lapis vows revenge and sets out to overthrow the tyrannical overlords. A dark Korean fantasy of rebellion and revenge.