Very nice blue hue. Quartz grows in many environments along with many different minerals. These
minerals, as well as watery solutions and gas bubbles, can be enclosed by the
growing crystal. Many minerals that would otherwise be chemically altered or
dissolved when the local conditions change are protected from aggressive
chemical agents when they have been embedded inside quartz crystals. Likewise,
fragile minerals that are very soft, crumbly or that occur as fibers or thin
needles survive inside. Other partially embedded minerals might get dissolved at
a later stage and leave ghost like hollow forms behind.
There are basically three ways inclusions can "get into the crystal":
The minerals have formed before the quartz. The growing quartz crystals
engulfs them and the original form of the included minerals is preserved. These
inclusions are called protogenetic. Included fibers than run through the
entire crystal at random orientations are typical examples.
Quartz crystals and the included minerals grow simultaneously. These
inclusions are called syngenetic. The shape of the included minerals
often deviates from the typical forms and habits that develop during unhindered
growth. Crystals may be distorted beyond recognition and a non-destructive
identification may pose a real problem even for a mineralogist. Sometimes
inclusions cause the formation of phantoms.
Here the quartz crystal might have been partially encrusted by another mineral
when growth halted transiently and continued later, such a case could be
considered a syngenetical formation that got overgrown.
Minerals can get into another mineral by exsolution. When the
conditions during crystal growth allowed the incorporation of elements into the
crystal lattice that are incompatible with the crystal structure at different
temperatures or pressures, these elements may separate from the lattice to form
new minerals once the conditions change. Such inclusions are called
epigenetic. Very often these inclusions are specifically oriented with
respect to the crystallographic axes of the main crystal. The best example of a
quartz with epigenetic inclusions is rose quartz.
Since the included minerals, liquids, and gases are well protected from
chemical alteration, quartz inclusions open a window to the past to the
scientist. Many crystals carry inclusions, but quartz has a simple chemical
composition and does not complicate the analysis of the included material too
much and does not interfere with substances used in chemical tests.
When the studied crystals are large and grew slowly - like rock crystals from
alpine-type fissures - one can even observe systematic variations in the
composition of the material that has been included during growth. In the central
oldest part of the crystal, for example, the salt content of liquids might be
higher, while the outer part of the crystal might contain more carbon dioxide.
Inclusions can also be used to estimate the temperature at which the crystals
formed.