Slate is a fine-grained, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite along with, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zircon. Slate can be made into roofing slates (sometimes called roofing shingles in the United States), because it has two lines of breakability: cleavage and grain.

PHYSICAL PROPERTIES OF SLATE
Hardness 2.5 to 4.0 on Moh's scale
Modulus of Rupture 450 to 850 Kg/cm2
Compressive Strength 170 to 240 Kg/cm2
Specific Gravity 2.65 to 2.80
Porosity Low to very low
Water Absorption 1.0 to 1.5%
Weather Impact Resistant
Corrodibility 0.4 to 0.7%

CHEMICAL PROPERTIES OF SLATE
Chemically they are Multi-Mineral rocks made of an aggregate of minerals and colloidal substances. Essentially mineral composition includes members of Mica group commonly sericite & muscovite, Clay group commonly paragonite, chlorite & kaonilite. The accessory minerals are oxides, calcites & minor amounts of quartz and feldspar along with subordinate amounts of ferromagnesian minerals. Free silica is normally poor. It is resistant to acids and staining.