Flagstone rock is a form of Sandstone.  Flagstone is a sedimentary rock that is split into layers along bedding planes.  It is a flat stone.  The name “flag” derives from Old Norse flagg meaning slab or chip.  Flagstone is usually a form of sandstone composed of feldspar and quartz and is arenaceous (medium) in grain size.  The material that binds flagstone is usually composed of silica, calcite, or iron oxide. The rock color usually comes from these cementing materials.  Typical flagstone colors are red, blue, and buff, though exotic colors exist.

Sedimentary sandstone from Arizona has been used in flagstone patios since the early 1900s, so there’s been a long time to test its suitability.  Lightweight, pale color and resistance to heat absorption has made it a popular choice in the Southwest where pool decking can become impossibly hot in the summer. However, in colder climates, the porosity of this material makes it problematic too, and in many conditions homeowners are redoing their patios with harder flagstone alternatives such as metamorphic quartzite.

Traditionally, Flagstone is used for paving slabs or walkways, patios, fences and roofing.  In the 13th century Anglo-Saxons used flagstones as flooring materials in the interior rooms of castles and other structures.  Lindisfarne Castle in England and Muchalls Castle (14th century) in Scotland are among many examples of buildings with surviving flagstone floors.  Portage Park in Chicago is known for its flagstone decorations

Flagstone is quarried in places with bedded sedimentary rocks that have breaks along bedding planes or with fissile bedding planes.  An example of a true flagstone is Arizona Flagstone mainly quarried from the Coconino and Prescott National Forests.

The Building Industry frequently interprets flagstone as a generic term used to describe natural stone that is suitable for horizontal planes that will be walked on.  They list basalt, andesitic, bluestone and quartzite as “flagstone”.  Although these are not true sedimentary flagstones, they solve problems caused from years of use on traditional sedimentary flagstone, such as flaking, displacement from frozen ground, pool chlorinating damage, erosion, water stains and discoloration.