Alexandrite

Mellochrysos is a greenish form of chrysoberyl which appears reddish under non-natural light (including wizard's light spells). This transparent gem is usually facet-cut and mounted as a pendant or earrings. Alexandrites are used in producing items which provide good omens (such as luckstones, though many are used for lodestones as well). A typical specimen has a base value of 100gp.