What are gemstone inclusions?
Inclusions are solids, liquids, or gases that became trapped in a gem while it was forming. A gemstone may contain more than one type of inclusion.
Aren’t inclusions bad?
While it’s true that visible inclusions decrease the beauty and value of many types of gemstones (diamonds being the best known example), some gemstones with inclusions are quite desirable. Some gemstones wouldn’t even exist if it weren’t for inclusions.
In addition, inclusions provide important information to gemologists. Some inclusions are only found in specific gems and others help differentiate natural and synthetic (man-made) gemstones. Inclusions can provide information on what region or even what mine a gem came from.
What gems with inclusions are desirable?
There are many types of included gems, but only a relatively small number are considered beautiful and tough enough for jewelry and are widely available. I’m going to give examples in 2 categories: inclusions that are the main attraction and phenomena caused by inclusions.
Inclusions as the Main Attraction
Amber, which is fossilized tree sap, may have inclusions of plants or animals. Amber with good quality inclusions can command very high prices.
Quartz is one of the most common host gems for inclusions, likely because it is one of the last minerals to crystallize during the cooling of molten material.
Rutilated quartz: Rutile’s needle-like crystals are commonly gold, black, or red. Rutile inclusions may occur as a few individual needles, throughout a stone, or in clusters. Occasionally golden rutile radiates out from a central hematite inclusion in a very desirable star pattern.
Tourmalinated quartz: While tourmaline comes in a rainbow of colors, tourmalinated quartz used in jewelry usually contains black crystals and can look similar to rutilated quartz. The cross section of a tourmaline crystal intersecting the surface of the quartz may show a characteristic modified triangular or hexagonal shape.
Phenomena Caused by Inclusions
Several of the phenomena seen in gemstones are caused by inclusions that may be invisible or barely visible to the naked eye. Some phenomena such as asterism and cat’s eyes require the gem be cut as a cabochon and properly oriented to show off the effect.
Asterism is the name for the 4, 6, or 12 rayed star effects seen in gems including sapphire, ruby, diopside, quartz, and others. It is caused when needle-like inclusions, commonly of the mineral rutile, but also ilmenite or magnetite, crystallize in orientations that create the star.
Chatoyancy is the silky sheen or band of light seen when light shines on parallel bundles of needle-like or fibrous minerals or hollow tube inclusions within the gem. The “cat’s eye” effect is a type of chatoyancy. Some common chatoyant gems are chrysoberyl and tiger’s eye. Other gems including beryl and tourmaline may show chatoyancy when the right inclusions oriented correctly are present.
Aventurescence (sometimes called Schiller effect) is caused by small flat inclusions that reflect light. Common gems with aventurescence include aventurine (containing fuschite, a green mica), and sunstone (copper, hematite, and/or ilmenite).
Learn more about phenomena in gemstones here.
There are many other beautiful and unique gems with inclusions. I’ve only highlighted some of the more common varieties here, but I hope you have a new appreciation of the beauty that can be contained within a gem!
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