The Queen of Amaranth

(from ebookjapan):

The Queen of Amaranth, a legendary diamond that brings a glorious fate and eventually a dark death to those who possess it.

Mara Green, a young girl who happens to have this diamond in her possession, is catapulted up the ladder of happiness to the point where the crown prince of a small country beckons her to his bedside.

However, unexpected misfortune befalls Mara at the top, and her life is threatened.