What Is the Luminance for Granite Rock?

Luster

• Luster refers to a mineral’s reflection of light. This quality is most broadly differentiated into metallic or non-metallic. The latter category includes a host of somewhat subjective grades. These include “greasy,” “oily,” “vitreous” (or glassy), “adamantine” (or brilliant, like a gem), “pearly” and the like. Luster is judged from cleavage surfaces – points along which some minerals easily dismantle – or a newly broken edge.

Granite Essential Minerals

• A given rock is partly defined by so-called “essential minerals,” the presence or absence of which can categorize it as one or another type. The essential minerals that help classify granites include quartz, potash feldspar and plagioclase feldspar. Quartz tends to have a vitreous or greasy luster. Potash feldspars are often vitreous, as are plagioclase feldspars, though along cleavage surfaces those may be pearly. In general, the crystal structures of quartz tend to give off the “twinkle” of a granite chunk considered under heavy sunlight, while feldspar minerals in the same conditions may have a crisp, reflective sheen, according to Charles W. Chesterman’s “The Audubon Society Field Guide to North American Rocks and Minerals.”

Granitic Rocks

• The word “granite” may refer to a very particular and common brand of igneous rock, but it also serves as an umbrella title for a group of related rocks of similar origin and composition, including monzonite and granodiorite. The differing concentrations of minerals affects the luster and general appearance of these cousins. Syenite, for example -- a much scarcer rock than common granite but often lumped with it -- contains little quartz.

Glacial Polish

• The veneer of polished granite makes it a popular choice for countertops, headstones, statues and other stonework. Just as striking is the wilderness sheen of glacially polished granite, famously showcased in the Sierra Nevada of California. This impressively scenic range stems partly from a massive granitic batholith, a mass that forms when magma intrudes beneath the Earth’s surface and is later exposed through erosion of surrounding layers. During the Pleistocene, the Sierras were heavily glaciated. The heavy sheets of ice, riddled with abrasive boulders and rock fragments, scoured the granite bedrock beneath, smoothing its surface. By the geological calendar, the 10,000 years elapsed since the Sierra glaciers retreated is barely a wink, and the “glacial polish” is still fresh on the landscape. So lustrous is the ice-scraped rock of the high country that it can be nearly blinding in full sun.