First and foremost, we need to look at the quality and strength of the color change. A strong change where the colors are bright and attractive under any kind of light is most desirable. Many stones are either a beautiful green in daylight or a nice pinkish red under incandescent light but they are less often attractive under both light conditions.
The ideal alexandrite is one with a medium tone and intense or near vivid colors; blue-green in daylight to rich ruby red or purple red under incandescent light or candlelight. Alexandrites that bleed, or those in which one can see the two colors at the same time may be less desirable but since light sources are often mixed anyway, the phenomena is quite common. Grayish and brownish tones are common in many Alexandrites and these stones are considered to be less valuable. A fine stone displays a distinct and dramatic color change where both colors are bright and attractive.
After color change, clarity, cut, and size are the next most important considerations. Clarity is important because cleaner stones are always more transparent and more brilliant as a result. Even the best alexandrites are expected to contain some contain inclusions, but for resale or investment, cleaner stones are always easier to sell. The cut is important because it is the proportions and finish which most affect the overall brilliance and appearance of the stone. Although perfect cutting is expected for most gemstones, alexandrite is one of those very rare gemstones where cut is often a secondary consideration to size. Since large stones are always more rare and valuable, the cut of an alexandrite is often a compromise between size and proportions. Ideally an alexandrite should be cut symmetrically with a good polish but weight retention is always an important consideration. After all, a large poorly cut stone can easily be recut into a more perfect smaller stone. Experienced cutters will whittle away at a stone removing the less attractive portions slowly cutting the stone and inspecting the result. Some stones are cut several times to reach the optimum color, size, shape, and brilliance.
When buying Alexandrite jewelry the buyer should always ask for a Certificate of Authenticity or a Gem Identification report from a reputed Gem Laboratory. Most reputable stores selling alexandrite will supply a certificate.