minerals
The definition is short and it does real work: it is what separates a mineral from a rock, and it is why a rock’s identity depends on how its minerals are arranged, not only on which ones are present.
A mineral is a naturally occurring substance with distinctive chemical and physical properties, composition and atomic structure.[1]
A rock is a solid collection of minerals — generally two or more, mixed together by geological processes.[1]
Quartz is silicon dioxide, and it is the index mineral for hardness 7 on the Mohs scale. Its reference density is 2.65 grams per cubic centimetre.[1,2]
Structure has consequences beyond hardness. Micas are sheet silicates whose platy grains can be aligned by pressure, which is what produces foliation in metamorphic rocks; the minerals of limestone are neither flat nor elongate, so no amount of pressure will align them.[1,2]
Magnetite is magnetic, and its presence in ocean-floor basalt is the reason the sea floor could record the polarity of Earth’s field as it cooled — and therefore the reason seafloor spreading was detectable at all.[1,2]
Granite is mostly quartz, feldspar and mica, present as interlocked crystals. A sandstone can contain the same three minerals — but as grains compacted and cemented into one another. Same ingredients, different rock.[1,2]
Often said
“A rock is just a big mineral.”
What the evidence supports
A rock is an aggregate of two or more minerals, and its texture — how those minerals are arranged — is part of what defines it. Granite and sandstone can share a mineralogy and remain entirely different rocks.
Supporting records: rock-definition · sandstone-mineralogy · granite-mineralogy
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.