Gemstones that are hard to find are valued more highly than those that are abundant. Treasured for its changeable colors, alexandrite is also valuable because it is so rare. Very little natural alexandrite exists. Other gemstones, although seemingly abundant, yield only a few specimens that are of usable quality. Gem-quality opals are difficult to find and very expensive.
Common opal is abundant and inexpensive.
Many rare and beautiful gemstones can't be used in jewelry because they aren't durable. They disintegrate or break too easily. To be durable, a stone must be stable, tough, and hard. Stable gemstones remain unchanged by heat, light, or chemicals. For example, strong light can cause yellow topaz to fade.