The selection of opal is based on factors such as color, position of eyeliner, shape and weight of gemstones. For high-quality cat's eye gemstones, the cat's eye line should be thin and narrow with clear boundaries; the eyes should be opened and closed flexibly to show lively light; the color of the cat's eye should be in sharp contrast with the background; and the cat's eye line should be located in the center of the arc. When purchasing, you should distinguish the opal from other gemstones and artificial cats according to the identification characteristics, and buy genuine opals.

Like other gemstones, the bigger the opal, the harder it is, so weight is still one of the basic elements in evaluating an opal. But the most important thing that affects the opal is the color, the condition and the evenness of the eyeliner.

The best colors for opals are very strong yellowish greens, tans and honey yellows, followed by greens and then slightly darker browns. Very white yellows and very white greens are worth less. The worst are variegated and gray.

The best opal eyeliner should be narrow, well-defined, bright, and centered on the gemstone. Regarding the color of eyeliner, some people like silvery forehead color, while others prefer golden yellow, while green and blue-white lines are more neglected. The body color is opaque gray, often with blue or blue-gray eyeliner. However, whatever the color, it is important that the eyeliner contrasts with the background and appears clean. The eyes should be able to open wide, the bigger the better, and they should be sharp when closed.

Processing technology (especially the degree of symmetry) is also an important factor in evaluating opals. In order to make full use of the original stone, the purpose of leaving it thick is to increase the weight and sell more money. But the result backfired, the asymmetry seriously affected the beauty of the cat's eye, and no one offered high prices. The correct way is to keep the appropriate thickness below the waistline of the gemstone and grind it into a small arc. To keep the angle low, this will help to make the gemstone set firmly on the support.