Fine color combined with flawless transparency is the ideal beauty in many gem species but as gemstones form they may trap other minerals. These mineral inclusions sometimes detract from the beauty of the stone but they may also form the chief attraction in some gems. Inclusions cause the color spangling in aventurine quartz and sunstone feldspar and the cat’s eyes and stars that gleam from some chrysoberyls and sapphires. To many people inclusions are merely flaws that reduce the value of a gemstone bot to the mineralogist and gemmologist they can reveal the gem’s identity, how it formed, and even the source locality. Since the appearance of synthetic gems, inclusions have acquired a greater importance, often providing proof of a natural or artificial origin. Most inclusions can be seen with a hand lens but are best studied under the more powerful magnifications of a microscope. Many gems contain small crystal inclusions usually of different mineral species from the host gem. These can give valuable clues to the temperatures, pressures, and rock types in which they formed. For example emeralds from many localities contain mica flakes derived from the mica schists in which they were formed. Colombian emeralds may contain distinctive three phase inclusions. These consist of jagged cavities containing saline liquid, a salt crystal, and a gas bubble. Inclusions in diamonds may provide information both about their origin and about the mantle rocks in which they formed. The ages obtained from certain mineral inclusions indicate that some diamonds formed over 3,000 million years ago and the youngest probably formed 900 million years ago. Crystal inclusions may have a good crystal shape or they may be rounded. Innumerable rounded apatite and calcite crystals form the distinctive internal granular texture of much hessonite garnet. Slender hollow tubes and needles like crystals of rutile, hornblende, and asbestos occur in many gems. Often developed parallel to one or more crystal directions when abundant they give rise to cat’s eye and star effects. Fractures and cleavages may develop in minerals as a response to stresses during their crystallization or in later Earth movements. The lily pads seen in many peridots are stress fractures that develop around chromite crystals or other crystal inclusions. Moonstones sometimes contain insect like structures, which are seen to consist of small cleavage cracks and are quite distinct from the real insects trapped in amber. Fractures may become partially healed during subsequent episodes of heating and alteration of the host rock and many such fractures retain pockets of trapped fluid as in sapphire feathers.