Slate is a fine-grained metamorphic rock derived from shale, a sedimentary rock composed of clay minerals. It is characterized by its planar, foliated structure, which allows it to be easily split into thin, flat sheets. Slate is often gray, but it can also be found in shades of green, red, or purple, depending on the mineral composition and the original shale’s depositional environment.

Mineral Makeup

The mineral makeup of slate is primarily composed of clay minerals, such as illite, chlorite, and biotite, which have undergone metamorphic changes due to heat and pressure. Other minerals present in slate may include quartz, feldspar, and calcite. The specific mineral content influences the rock’s color, texture, and overall properties.


Slate forms through the low-grade metamorphism of shale under relatively low temperatures and pressures. During this process, the clay minerals in the shale are altered, realigned, and compacted, resulting in the formation of a more dense and durable rock. The planar fabric of slate, known as slaty cleavage, develops perpendicular to the direction of the compressive stress applied during metamorphism.


Slate is found in many parts of the world where shale deposits have undergone regional metamorphism. Notable occurrences of slate include the Slate Belt in the northeastern United States, the Lake District in England, the Ardennes region in Belgium, and parts of the European Alps. Slate is often associated with other metamorphic rocks, such as phyllite, schist, and gneiss.


Slate is believed to possess metaphysical properties associated with transformation, stability, and clarity. As a metamorphic rock, it symbolizes the power of change and adaptation. Slate is also thought to provide grounding energy, promoting a sense of balance and mental focus during times of upheaval or uncertainty.

ClassMetamorphic (Foliated) 
Mineral MakeupPrimarily clay minerals (illite, chlorite, biotite), with quartz, feldspar, and calcite
LusterDull to slightly shiny
Hardness (Mohs)3-4
StreakWhite or colorless
ColorGray, green, red, or purple
CleavagePerfect, splitting into thin sheets
Specific Gravity2.7 to 2.8