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. Toughness refers to the brittleness of a gemstone. The more brittle it is, the easier it will break. Hardness, which is different from toughness, is the ability of a gemstone to resist scratching and general wear. Diamonds are the hardest of all gemstones.