Hydrogrossular garnet is an especially rare gemstone. It's defined by a bright green color, and most commonly found with black inclusions. These inclusions are chromite, and the green color is caused by a reaction in the stone when the chrome is introduced to water. The green coloring has likened the stone of jadeite in the past, and because its hardness has made the stone perfect for beads and cabochons, it has often been used as a substitute.