Valuable Gemstone Inclusions
It's a common misconception among gemstone buyers that inclusions are a bad thing. This view is fostered by simplified gem buying guides that offer the advice that the fewer the inclusions, the better. Though this advice is certainly true for diamonds, it is not always the case in the world of colored gemstones.
There are very few colored gemstones which are internally completely "clean." Gemologists use the term inclusion to refer to foreign matter or irregularities of the crystal lattice in a gemstone. It is worth noting that gemology uses the term inclusion rather than "defect" or "flaw" to refer to these phenomena. For often they are not flaws at all. Sometimes they actually contribute to the beauty or value of the gem. In virtually every case they provide important clues to the origin or formation of the stone.
One of the most famous and valuable gemstone inclusions is the organic matter trapped in amber. Amber is the fossilized, hardened resin of the pine tree, formed about 50 million years ago. Specimens with included insects or plant materials are especially valuable.
Rutilated quartz is another gem variety that draws its value from its characteristic inclusions. Rutile is the mineral name for natural crystals of titanium dioxide. While most varieties of transparent quartz are valued most when they show no inclusions, rutilated quartz is valued specifically for the lovely patterns formed by the delicate golden needles of rutile inside it.
Inclusions of rutile needles add value to another group of gemstones -- those that exhibit asterism (the star effect) or chatoyancy (the cat's eye effect). When aligned needles intersect each at the right angle, a star sapphire or ruby will display six white rays of light that meet in one point and dance over the gem's surface when viewed from different directions.
Another group of gems depend on characteristic inclusions for their distinctive look. For example, the pyrite inclusions in lapis lazuli give the venerable royal blue gem its unique brassy shimmer. Jasper, a form of chalcedony, contains up to 20% foreign material that provides its trademark multicolored, striped, spotted or flamed appearance. Moss opal carries dark green dendrites in moss-like patterns. Without the dentritic inclusions, it is just a common opal of no particular interest.
Even in the transparent gems there are inclusions which are especially valuable. Fine rutile silk in corundum from Kashmir and Burma lend especially fine sapphire and ruby a velvety color that is highly prized. A unique horsetail inclusion in rare Russian demantoid garnet is regarded by collectors as the most reliable indication of Russian origin, since the more common Namibian demantoid lacks this unusual inclusion.