Zircon

Composition: Zironium silicate, often with some hafnium ZrSiO4
Hardness: 7.5

Metaphysical Properties: Crystal of virtue. Continuity and balance. Promotes unions, be it emotional, spiritual, physical, or mental. Symbolizes innocence, purity, and constancy.
Corresponding Astrological Signs: Virgo, Sagittarius, Leo