ASTM standard C119 defines slate as a "microcrystalline metamorphic rock derived from shale and composed of mica's, chlorite and quartz. These minerals have a sub parallel orientation and thus impart strong sleacage to the rock, which allows the latter to be split into thin, but tough sheets.

Genuine Slate is a metamorphic stone with very distinctive properties and performance criteria. Slate counter tops, Slate sinks and Slate flooring are just three examples of how genuine slate may be used.

Regardless of the name given to a type of stone, or the source location of that stone, the material should be tested to determine it's suitability for interior or exterior use and the type of application.

AMST Designation C 629-89 Slate Dimension Stone*

Property	Test Requirements	Classifications	ASTM Test Methods
Absorption	0.25 0.45	1 Exterior 11 Interior	C121
Modulus of rupture Mpa Across Grain	9000[62.1] 9000[62.1]	1 Exterior 11 Interior	C120
Along Grain	7200[49.6] 7200[49.6]	1 Exterior 11 Interior
Abrasion resistance	8.0 8.0	1 Exterior 11 Interior	C241
Acid resistance	0.015[0.38] 0.025[0.64]	1 Exterior 11 Interior

*Reprinted with permission from the Annual Book of ASTM standard, copyright American Society for Testing and Materials. Slate standards C119, C629, C406 may be purchased from ASTM, 100 Barr Harbor Drive, West Conshobocken, Pa. 19428-2959