Sapphire form in rocks that geologists call undersaturated, or rocks that have no free quartz. Spinel is another gem that forms in undersaturated rocks. Modern gem testing procedures can easily separate the two different minerals. The colors of sapphires and rubies are controlled by trace elements that are included in the crystal lattice in amounts usually not exceeding about three percent. Boule of synthetic sapphire.
Boules develop split upon cooling.
Synethetic sappire cabochons. Blue glass, a common sapphire substitute. Sapphire-bearing alluvial gravel, Montana. Miner digging sapphire-bearing gravel, Montana. Sized and sorted gravel concentrate that may contain sapphires.