for this purpose, classifying it as basalt if it has a color index of 35 or greater.

The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content. The average density of basalt is 2.9 g/cm³, compared, for example, to granite's typical density of 2.7 g/cm³. The viscosity of basaltic magma is relatively low--around 10⁴ to 10⁵ cP--similar