A treated emerald with a tinted oil to enhance its color is most likely of lesser quality to begin with, and would be worth at least 50 percent to 75 percent less.

The price differential between a natural and a moderately enhanced sapphire is not as vast as rubies and emeralds. Matlins estimates a high-quality natural Ceylon sapphire, considered to be among the most desirable, costs between 30 percent and 50 percent more than a moderately heat-treated stone of similar size. Ceylon sapphires are a lighter shade than the crystal Burmese sapphire, which is usually a deep indigo, or the velvety-toned Kashmir variety. Ceylon sapphires tend to be a cornflower blue, though some verge on pastel. Heat treatments often reduce transparency and muddy the crystal effect, yet naturally darker sapphires are often treated to lighten the color, while those that seem too pale might be X-rayed to intensify the color.

Despite the high price tag of natural gems, Matlins believes they are still a good investment. "At this point, even the most sophisticated consumers are generally not aware that most stones are treated," she observes. "As they become aware of treatments, they will start searching for natural or minimally treated stones, and that will drive up the prices in the years to come."