Iron-stained milky quartz

White to translucent, irregularly crystalline mass with a vitreous to slightly greasy luster. Quartz has a Mohs hardness of 7, no true cleavage, conchoidal fracture, and a specific gravity of about 2.65. The tan, orange, and brown coloration is probably caused by iron oxides or hydroxides such as hematite or limonite coating fractures and grain boundaries. Individual crystals are not clearly developed in the photograph, so exact identification of minor minerals is uncertain.

Most likely formed as quartz-rich hydrothermal vein material, precipitated from hot, silica-bearing fluids filling fractures in surrounding rock. The iron staining developed later through weathering and oxidation of iron-bearing minerals or fluids. Quartz veins can form during many geological periods, from Precambrian to recent, so an age cannot be determined from the image alone.

Common quartz is used in glassmaking, ceramics, engineered stone, abrasives, foundry work, and silicon production. Attractive pieces are sold as mineral specimens or used in lapidary work, although this rough, heavily stained specimen is more suitable for collecting than fine jewelry.

Milky quartz appears cloudy because of microscopic fluid inclusions, fractures, or tiny mineral inclusions. Iron oxides commonly stain quartz orange, yellow, red, or brown. Quartz is one of Earth's most abundant minerals and is a major component of granite, sandstone, and many hydrothermal veins.

In the field, quartz can usually be recognized by its white or glassy appearance, lack of cleavage, conchoidal fracture, and ability to scratch ordinary glass or steel. A streak test should be white. The specimen may fizz weakly or not at all in dilute hydrochloric acid; strong fizzing would suggest abundant calcite rather than quartz. The image is also compatible with a quartz-rich pegmatite or a quartz-calcite vein, so a hardness test, acid test, and close inspection of cleavage are recommended for confirmation. Quartz veins are common in mountain belts, fault zones, granites, pegmatites, and hydrothermal mining districts worldwide.