Ethiopian Opal Classification
Colorless, White, Yellow, Orange, Brown
Key tests are play-of-color, refractive index and fluorescence. Opals might phosphoresce, and this is a strong indication of its natural origins, as synthetic or simulant opals do not phosphoresce.
Many Ethiopian opals are hydrophane opal which is porous and will take on water. The SG of stones can be variable due to soaking in liquid or due to the humidity of surrounding atmosphere.