Inclusion.

An inclusion is anything that found itself caught inside a crystal while the host was growing. It can be another mineral (a solid inclusion), a tiny pocket of fluid left over from the growing solution (a fluid inclusion), a bubble of gas (a gas inclusion), or even a thin slice of the surrounding rock. Mineralogists also distinguish protogenetic inclusions (older than the host, swept up as it grew), syngenetic inclusions (formed at the same time), and epigenetic inclusions (introduced later, often along a fracture). Inclusions are common, often beautiful, and frequently the most interesting thing about a piece.

Named varieties and their guests

Some of the best-known crystal varieties are named for the inclusions they carry. Rutilated quartz holds golden, copper, or silver needles of rutile (titanium dioxide), which form during high-temperature growth and can grow as a starburst from a hematite seed (the formation called "Venus hair"). Tourmalinated quartz holds black tourmaline rods (schorl) suspended in clear quartz like ink in glass. Lodolite or "garden quartz" carries cloudy ghosts of chlorite, feldspar, and other minerals that look like landscapes inside the stone, sometimes with tiny pyrite cubes catching the light. Phantoms are an older crystal layered inside a younger one, marking a pause and resumption of growth, and are common in amethyst and clear quartz. Demantoid garnet from the Russian Urals is prized for its "horsetail" inclusions of fine fibrous chrysotile, the only gemstone where an inclusion makes the material more valuable, not less. Hematite, mica, dumortierite, ajoite, papagoite, and even small water bubbles (sometimes containing a moving gas bubble, called a two-phase inclusion) all show up regularly.

Some inclusions carry information far beyond their visual appeal. The hexagonal silk in Burmese ruby is fine rutile fibres that scatter light into the soft glow gemmologists call "ruby fluorescence." The three-phase inclusions in Colombian emerald (a liquid, a gas bubble, and a salt crystal in one tiny capsule) are diagnostic enough to be used to confirm origin. Sapphire inclusions can date a crystal's growth, since the pattern of mineral guests records the chemistry of the fluid it grew from.

How buyers should read them

Inclusions tell a geological story: what else was present in the fluid when the crystal grew, what temperature and pressure conditions were like, and how the surroundings changed over time. They are also a useful authenticity check. Glass and synthetic stones often have either no inclusions at all (suspiciously clean) or the wrong kind, such as round gas bubbles in glass that natural quartz almost never shows. A loupe held to a stone is often the cheapest way to tell flame-fusion ruby from natural ruby, because the synthetic shows curved growth bands and gas bubbles, while the natural shows angular silk and crystalline guests.

A common misconception is that an included stone is a flawed stone. In faceted gem grading, where clarity is one of the four Cs, inclusions do reduce the price. In specimen and crystal collecting, the opposite is often true: a piece of clear quartz with a perfect rutile starburst is worth far more than the same quartz without it. In modern crystal practice the contents are often read as adding character or carrying their own meanings: rutile for clarity of thought, tourmaline for protection layered into amplification, chlorite for grounding into growth. A clean stone is not better than an included one. They are different kinds of beautiful.