What Is Quartz?

Quartz, Pure and Impure

Quartz is the second most common mineral on the planet after feldspar.

In its pure form quartz is colorless. However, when impurities are present the color of quartz changes.

When impurities turn quartz purple, it is known commonly as amethyst. Likewise, when impurities turn quartz yellow, it is commonly known as citrine. Pink quartz is known as rose quartz, and so on.

As the differently colored quartzes are essentially the same stone they share gemological properties: The same crystalline structure, the same Mohs hardness (7 Mohs), the same luster etc.

Macrocrystalline, Microcrystalline and Cryptocrystalline Quartz

One of the principle sources of confusion about quartz stems from attempts to sub-categorize it.

Originally quartz was split into three subcategories: Macrocrystalline, Microcrystalline and Cryptocrystalline.

Macrocrystalline quartzes have larger crystals than micro- and cryptocrystalline quartzes.

Microcrystalline quartzes are those with a crystal structure which is discernable through an optical microscope whereas cryptocrystalline is so small that it is not discernable through an optical microscope.

Fortunately, this subcategorization has no been simplified: macrocrystalline quartz is now known as "quartz" and the micro and crypto varieties are now known as "chalcedony".

Quartz and Chalcedony

The big quartzes include stones like amethyst and rose quartz. The small quartzes - that is, the chalcedonies - include agate, carnelian and jasper.

Agate

Agate is a further sub-classification of the quartz family. It is a type of chalcedony that has many different forms and presentations.

Agate is usually formed in cavities. These cavities are usually volcanic rocks, though they may well also be formed in cavities in wood or coral.

When the cavities are filled with silica-rich water, eventually the water crystallizes.

The process of crystallization may produce a number of different agates:

Banded agate

Banded agate feature bands of colors or hues. The colors of the bands are determined by impurities.

Fire agate

Fire agates have a 'fire' effect created by bubbles on a reddish-brown stone. The fire effect is caused by the diffraction of light in iron oxide layers in the quartz.

Dendritic agate

Dendritic agates feature tree-like patterns which give the agate the impression of depicting a natural landscape.

Moss agate

Moss agate is milky with green or brown moss-like patterning created by inclusions of green chloride or brown manganese.

Petrified agate

Petrified agate is created when organic substances - wood or coral, say - become fossilized.

Over the centuries, the pressure exerted onto the organic substances activates their slow transformation into minerals.

Specifically, as the wood or coral decomposes, it is replaced by minerals.

As quartz is one of the most abundant minerals on Earth, it stands to reason that it has been subject to numerous classifications.

These classifications have made the understanding of quartz, at times, harder than it should be.

Essentially, quartz can be understood as a tree like structure with quartz being formed of two major categories - Bigger quartz and smaller quartz. For sake of convenience bigger quartz is simply "Quartz" and smaller quartz is "Chalcedony".

Chalcedony has many subtypes, one of which is a large subtype known as agate.