found in glacial gravels and upper Mesozoic sediments in southern Illinois E. Hardness greater than 7 (cannot be scratched by quartz) C—red, brown, H—7.5 Irregular grains or Garnet Group yellow, green, Cl—poor masses; sometimes as (Ca,Mn,Fe,Mg)₃ black, white F—even 12-, 24-, and 36-sided (Al,Cr)₂(SiO₄)₃ crystals; abundant in glacial sands and Lake Michigan beach sands; common in metamorphic rocks ROCK IDENTIFICATION KEY SAMPLE Scratch with a knife and apply dilute acid (HCl) If rock does not scratch, go directly to I, II, or III No effervescence or very slight effervescence I Coarse-grained (p. 37) II Fine-grained (p. 38) III Organic (p. 39) Slight effervescence gray, light gray, white, or brown: Dolomite Vigorous effervescence Composed of pebbles that effervesce Rounded pebbles: Limestone conglomerate Angular pebbles: Limestone breccia Composed of crystals of calcite, fossil shells, or oolites: Limestone Composed of banded layers of crystalline calcite; commonly found in caves, forming stalactites and stalagmites: Travertine Large amount of insoluble residue left on acid-treated surface Individual grains seen with unaided eye: Calcareous sandstone Individual grains not seen with unaided eye: Calcareous shale Composed of porous or cellular mass of calcite; commonly found near springs and waterfalls: Tufa I COARSE-GRAINED ROCKS