Moonstone is the name given to some gem varieties of feldspar (Na,Ca)Al₁₋₂Si₃₋₂O₈, KAlSi₃O₈ with a specific gravity 2.56 to 2.60. Monoclinic crystal system. Hardness 6-6.5 on the Mohs scale
Common moonstone is usually milky white, but can come in orange (peach), and black; all colours of common moonstone can display a pearly, milky play of light. Rainbow moonstone (also clear, milky white to beige) is the most highly valued variety, and the only type to display the blue flash that most people associate with moonstone. Rainbow moonstone can also in rare occasions display flashes of orange, red and green but only where the crystal structure grows to meet. All varieties of moonstone need to be cut on a 30 degree angel to the grain to the stone in order to show their colour, so usually only one face will display the play of light or flash of colour.