Since I mentioned inclusions a few times last weekend and showed off a lot of rutilated quartz I thought I’d explain it a little bit here.
In mineralogy* inclusions are materials trapped in minerals, normally during its formation. They’re often other minerals, but sometimes they are liquids or gases. Technically the insects and plants found in amber are inclusions too.
Inclusions are not the same thing as flaws.
Sometimes they are considered flaws (in diamonds), sometimes they’re an expected element (as in emeralds), sometimes they’re considered a different variant of a gemstone (as in star sapphires), and sometimes they raise the value of a stone (in rutilated and tourmelated quartz).
Excepting diamonds, inclusions are generally judged on their overall impact on the presentation of a stone.
Inclusions are pretty common in minerals and not always visible to the naked eye. Gemologists used to assume that inclusions had to be older than the mineral around them. Which seems to make sense, but isn’t always the case.
Some are older minerals that were surrounded by the formation of the other mineral (the host crystal). Others formed at the same time but grew slower so were consumed by the growth of the host crystal, and some actually form later when mineral bearing liquid enters crystals through cracks.
Rutile inclusions form inside the crystal as it cools.
Rutile is a mineral composed mostly of titanium dioxide, with a smaller percentage of different metallic elements. It comes in a wide range of colors and it gets its name from the red version. There are reds, coppery colors, yellow-gold and black.**
It is normally the golden and copper that you find as inclusions in quartz.
The two things most looked for in rutilated quartz is the clarity of the quartz itself and rutile stars.
Rutile stars are six sided stars that align in bundles around a hematite inclusion. Apparently they rarely occur in ways that can be cut to show off the star.
If not forming stars, generally finer rutile inclusions are preferred.
Dense concentrations of very fine rutile needles in sapphires and rubies make the stars (asterisms) that are so desirable. Rutile inclusions can also cause a sheen sometimes referred to as silk sapphire or ruby.
If titanium dioxide sounds vaguely familiar, that’s probably because it’s used in pigments for a bright white (white that’s much safer in oil paints than lead white, but also doesn’t have the same feel) and in sunscreen. (I keep hoping it wasn’t the pretty crystals that got pulverized to make these!)
*(In gemology inclusion is often used to refer to any irregularity in the crystal structure of a gemstone, since it doesn’t have the same derogatory connotation as flaw. But it does make things confusing!)
**Rutile can be black, however, most of the time something is labeled black rutilated quartz it’s actually tourmelated quartz. Same deal, but in this case the inclusion is tourmaline instead of rutile.