While nearly all gem alexandrites are beautiful stones, the value of fine alexandrite is closely tied to the strength of each stone's color change. While this might be idealistically described as changing from green to red, few stones come close to this. The change should be easily discernible under the correct lighting conditions. If there is no change, it becomes merely chrysoberyl. Like most other colored stones, alexandrite's value is also determined by its purity of color and degree of saturation as well as its clarity. Though as a type II gemstone, alexandrite is expected to show some inclusions to the unaided eye that shouldn't detract much from its beauty. When the inclusions are fine needles that are oriented in the same direction, a cat's eye effect can be seen, which is even more rare. Supply, availability and rarity also figure strongly in alexandrite's value. Alexandrite only forms when aluminum/beryllium and chrome (which rarely occurs in nature) come together, accounting for its rarity.
The Smithsonian Institution houses the world's largest faceted alexandrite, a Russian beauty weighing 66 carats. The crystal reported to be the largest gem-quality alexandrite ever found is the Sauer Alexandrite, a 122,400-carat uncut crystal that was found in Bahia, Brazil, in 1967.
Because alexandrite is known for its strong color-change ability, other color-change stones (like some garnet and spinel) are said to have "the alexandrite effect."
Before the advent of modern lighting, color change effects were rarely observed because one would have to wait for night or daylight, or change locations in order to observe the change. Few color change gems change dramatically enough to have their change noted, and those were very rare from the beginning. To correctly judge a change in color, gemologists separately use incandescent lighting and natural daylight. Daylight is often substituted with fluorescent lighting, but incandescent lighting can only be substituted with candlelight. Incandescent lighting – the old-style filament light bulbs - is increasingly scarce, but it is only in this light that the red to purple shades of alexandrite can be viewed. For proper grading, all other types of light must be removed. When both lighting is present, there is an indistinct mixing of color that does not show alexandrite to its best effect. This is true of all color change stones.