Pennsylvania Bluestone Flagstone – The Basics

With the amazing proliferation of manufactured paver and imported stone options, it is easy to overlook local flagstone for your patio project.  Flagstone is a generic term for flat stone usually used for paving applications such as patios and walkways. In Pennsylvania, we use the term, flagstone, to refer to Pennsylvania Bluestone.  This layered stone is found only in eastern Pennsylvania and parts of northern New Jersey and southern New York.    

Bluestone quarries produce a rich variety of stone which results from the variety in the stone as various depths in the quarry.

The upper crust of a quarry becomes Colonial wall stone used for dry-stacked or mortared stone walls.  In the next layer, the pieces of broken stone become increasingly larger resulting in what is commonly called garden path, or stepping stones.  As these pieces become larger and thicker, they are referred to as slabs, generally used for large natural steps or water features.

Once past these top layers, the most popular bluestone layers emerge.  First, irregular or stand up flagstone – large irregular pieces 1-3” thick.  These pieces are dense enough for paver applications but not yet dense enough to be cut into large rectangles.  Next, the stone becomes dense enough to be cut into huge cubes which separate naturally as they dry.  This is pattern flagstone, pieces cut into squares and rectangles usually from 12” x 12” up to 24” x 36”.  This natural clef flagstone is the most common patio product.  Pattern flagstone is sorted into 1” (which is really ½” – 1 ¼” thick) and 1 ½” (which is at least 1 ¼” but can be much thicker).

But, there is still denser stone in the quarry.  These final layers are cut into thermal pattern, treads, and steps.  In these densest products, a saw cuts the exact thickness desired producing near perfect thickness versus the variety caused by natural clef separation.

Pennsylvania bluestone is not always blue.  Depending on the minerals in the stone, bluestone may be true blue, lilac, or, most common, variegated or full color.  Full color flagstone offers a huge range of beautiful colors, each piece unique: blue, green, taupe, lilac, brown, and rust.  It is this beauty which generally draws homeowners to this amazing stone.

Irregular, pattern, thermal bluestone can all be used for patios, sidewalks, and other outdoor paving applications.  Even driveways are possible with the thickest densest stone.  Flagstone projects may be wet laid, i.e., set in mortar on a concrete base, or dry set (most common), set in screenings or stone dust in a process very similar to manufactured pavers.