As we stroll across cleared fields, our curiosity may be stirred by numerous white rocks resting atop the soil. We usually shrug them off as just plain quartz. Quartz is a mineral; occasionally it is massive enough to be called a rock. Red hematite and yellow limonite are minerals that may add colors to the fissures within quartz as well as discolor its surface.
The commonly found rock in most local fields is usually vein quartz, as seen in the top picture. The bottom picture shows a line of vein quartz where it originally formed, embedded in the decomposed, geologically ancient rock that is changing to soil. The hard quartz resists natural weathering, thus it is eventually left on the surface when the other surrounding materials erode away.
I remember in times past elementary school students' bringing their first box of rock specimens to be identified by a high school science teacher. Fooled by the differing appearances, those beginning students had often created a collection of variably colored “rocks” — but all were quartz!
See also further discussion of vein quartz.