Quartz Crystal Similar To That  Used To Create The Skulls

Quartz is one of the most common minerals in the Earth's continental crust. It belongs to the hexagonal

crystal system, and is made up of silica (SiO2) tetrahedra. Quartz has a hardness of 7 on the Mohs scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.

Quartz is one of the most common minerals of the continental crust and goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline (which includes Rock Crystal).

Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true.  While it is colorless and transparent when pure, when a tiny portion of the silicon atoms are replaced with iron, aluminum, manganese or titanium, the crystal can take on beautiful colors. Amethyst is violet quartz. Jasper is quartz with red, yellow, brown, gray or black coloring. Onyx and Agate are quartz with bands of color. Bloodstone is green with red spots. If a single streak runs through the quartz it might be called Cat's Eye, Tiger's Eye or Rutile.

Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several meters in length and weigh hundreds of pounds. These veins may bear precious metals such as gold or silver, and form the quartz ores sought in mining. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."

Quartz is a common constituent of granite, sandstone, limestone, and many other igneous, sedimentary, and metamorphic rocks.  All of these rocks are common throughout Mesoamerica and Andean South America.

Tridymite and cristobalite are high temperature polymorphs of SiO2 which occur in high silica volcanic rocks. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.

Piezoelectricity

Quartz is also a type of piezoelectric crystal that creates electricity through a process called piezoelectricity when mechanical stress is put upon it. One of the earliest uses for a quartz crystal was a phonograph pickup. Today, one of the most ubiquitous piezoelectric uses of quartz is as a crystal oscillator—in fact these oscillators are often simply called "quartzes" or "crystals". The same principle is also used for very accurate measurements of very small mass changes by means of the quartz crystal microbalance.

The BH Skull

Dating Crystal

We are not referring to going out on a Saturday night, but rather the ages of crystals.

Quartz was formed over millions of years, and as such it dates to the geologic era that created it.  However, micro-particles can adhere to an artifact that can aid in dating the object.  Usually the best way to date a stone artifact is chemical reaction, as well as context.

Unfortunately, in the case of our Crystal  Skulls, we have neither context, not chemical reactions, nor micro-particles.  All we have is a dubious verbal history.

There are no good ways of guessing how long ago a skull shape was carved or polished out of the quartz. Scientists have examined some of the skulls looking for tool marts, but this not always reliable for age or origin. It does not eliminate contemporary artists using ancient methods - thus eliminating almost of the so-called Crystal Skulls, except the primary two (and a small number of related crystal objects).