Tiny cracks on the surface. These can eventually break the stone.
Some opals absorb water and lose color temporarily. If you'd like to narrow this down, let me know: What is your budget range ? Do you have a favorite color (Fire red, ocean blue, etc.)? how to buy opals
Look at the side. Straight, glued lines mean it's a composite. Pattern and Shape The way the color is arranged affects the price. Harlequin: Large, distinct squares of color. Rarest. Pinfire: Tiny dots of color. Common. Flash: Large blocks of color that blink on and off. Tiny cracks on the surface
Used to make cheap Ethiopian opals look like Black Opals. etc.)? Look at the side. Straight