In the early twentieth century, George Barrow introduced the concept of Index Minerals to identify the progression of metamorphosis in rocks from his obseversation of Scottish metamorphic rocks. Index minerals are unique, i.e. read more
Simply because they are abundant and common. Quartz (or silica) exists in almost all types of rocks with varying proportions. Similarly, Calcite is a common rock forming mineral which occurs in both sedimentary rocks (e.g. limestone) and metamorphic rocks (e.g. marble). read more
Quartz and calcite are not opaques. Re #4: Calcite has a hardness of 3, not marble. A rock can bear more that one type of mineral each of which will have a different hardness. read more
Quartz and calcite are common minerals in the rocks around the world. Both minerals form in a variety of colors, such as purple, white, brown, gray and colorless, which at times makes them appear similar. read more