Black opals
Black opals should possess an abundance of red flashes, or pin point flashes, to be of first quality. If the reds are mixed with greens, they are also good, but broad, even colors are not so valuable. If the brown matrix is showing, the value of the stone is still lessened. Even smaller pieces of fine black opal are comparatively rare, that is, stones weighing three or four carats, and $140 (£50) a carat is not a high price for fine qualities. One should beware of black opal doublets, which are frequently seen in good class jewelry. They may be just as attractive as the natural stone, but their commercial value is far less.
In white opals, the most usual variety in use, the background is a milky tinge, whitish, bluish, or greenish, but the best specimens show such a wealth of red flashes that this bodyground is hardly seen. Stones which show red and green, and also only green, are considered good in quality, but the play of color must be strong and not reserved to parts of the stone only. Bluish stones are of not much value, and thdse which are almost all milky white without any color relief are unsaleable.