Moonstone is a feldspar gem: orthoclase with thin layers of albite inside. Light scattered between the layers gives a floating glow called adularescence.
Properties
| Refractive index | 1.518 to 1.526 |
|---|---|
| Birefringence | 0.005 to 0.008 |
| Optic character | Doubly refractive (biaxial) |
| Dispersion | 0.012 |
| Specific gravity | 2.58 |
| Hardness (Mohs) | 6.0 to 6.5 |
| Crystal system | Monoclinic |
| Pleochroism | None |
| Absorption spectrum | Not diagnostic |
Published values. They change a little from stone to stone. Our tools do not model moonstone yet. Sources: GIA · Gemdat · International Gem Society
In more detail
A blue sheen on a colorless background is a prized combination. Rainbow moonstone is not orthoclase: it is a variety of labradorite, another feldspar. Moonstone has poor toughness and two cleavage directions, so a hard blow can break it. Clean it only with warm, soapy water.
Related terms
- Cleavage: Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.
- Toughness: Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.
- Cabochon: A cabochon is a gem polished to a smooth, convex dome, with no facets. It is one of the oldest and simplest cuts.
- Chatoyancy: Chatoyancy is a single band of light that glides across a gem, like the slit eye of a cat. Light reflects from many aligned, needle-like inclusions.