The white mineral grains found in granite are feldspar. It is the most abundant mineral found in granite. Unlike granite, marble was never molten rock, but it may have been heated and squeezed enough for the limestone grains to bend and flow. Because marble is made entirely of calcite, it is much softer than granite. Slate is chiefly comprised of quartz and illite, with mica, calcite, and minor quantities of other various minerals.
Slates vary in color, most commonly seen in black, gray, red, purple, or green.
Red color is due to the presence of Hematite, while green colors are due to Chlorite.