Quartz is the most abundant mineral in the Earth’s crust. You can find quartz in sedimentary, igneous and metamorphic rocks, and in geological curiosities like geodes. While quartz has numerous varieties that vary in color and crystal type, Rock Crystal is a common term for the clear variety. Quartz is composed of silicon and oxygen, and has distinct physical properties that make it identifiable.
Observe the color of the crystal. Clear quartz may have small inclusions that look like a smudge in the crystal, but overall the crystal should be colorless and transparent.
Inspect the crystal shape. Quartz crystals are generally hexagonal prisms that terminate on each end with a six-sided pyramid. The entire crystal may not be perfect, depending on the how the quartz formed. Your sample may have only the barrel of the prism, and one pyramid or the end pyramid may appear to have only three sides.
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Scratch the surface of the crystal with a pocketknife to test the hardness. Quartz is harder than the blade of the pocketknife. Therefore, you should not see a scratch on the crystal surface.
Rub the crystal on a streak plate to test for the presence of a streak and its color. Streak is the color of a mineral in powdered form. Quartz will streak either white or colorless. Streak plates are approximately the same hardness as quartz, so you may see a white streak or simply scratches with little to no color.
Break the crystal with a hammer to test for cleavage and fracture. Quartz crystals do not have good cleavage, which is the ability of the crystal to break along smooth planes of weakness within the structure. Rock Crystal will fracture instead, showing a rough surface along the break. Often, the fracture will exhibit a swirl pattern, also known as a conchoidal fracture.
Calcite may look like quartz at times but will bubble in the presence of an acid. Halite may look like quartz but tastes salty.