Granite pebble

The specimen is a rounded, naturally worn pebble with a coarse, interlocking texture. It contains abundant cream to pink feldspar, translucent gray-white quartz, and scattered black to dark-green biotite or amphibole. Luster is mostly vitreous on quartz and pearly to dull on feldspar and mica. Individual mineral grains show cleavage, but the rock as a whole has no distinct cleavage. Typical granite hardness is approximately 6–7 Mohs; specific gravity is commonly about 2.63–2.75. The visible texture is phaneritic, meaning the crystals are large enough to see without magnification.

Granite forms when silica-rich magma cools slowly deep within the continental crust, allowing large crystals to develop. The rounded shape indicates later erosion and transport by water, ice, or sediment movement. The rock itself could be hundreds of millions to several billion years old depending on its source; its exact age cannot be determined from the photograph. Pink coloration commonly results from potassium feldspar, while dark minerals represent biotite mica and/or hornblende.

Granite is widely used as crushed stone, road aggregate, building stone, monuments, countertops, paving, and decorative landscaping rock. Small polished or naturally rounded pieces are sold as lapidary and mineral-collecting specimens, although ordinary granite pebbles usually have little jewelry value.

Granite is one of the most common continental crust rocks and is compositionally equivalent to volcanic rhyolite, but granite cools underground while rhyolite erupts at the surface. Its coarse crystal size is evidence of slow underground cooling. The specimen appears weathered and possibly water-worn, which can soften its original angular grain boundaries.

In the field, identify granite by its visible mixture of light quartz and feldspar crystals with scattered dark mica or amphibole, producing a speckled appearance. Quartz is typically glassy and lacks cleavage; feldspar is commonly pink, white, or gray and has two cleavage directions; biotite occurs as dark flaky plates. A steel knife generally does not scratch the quartz grains, while a fresh surface may fizz weakly or not at all with dilute hydrochloric acid. The image alone cannot completely exclude granodiorite, pegmatite, or a weakly foliated granite gneiss; a fresh broken surface, hand lens, hardness tests, and mineral proportions would improve identification. Common sources include continental mountain belts, exposed batholiths, glacial deposits, stream gravels, and commercial granite quarries worldwide.