Colorful, beautiful and easy to maintain. There are a lot of good reasons to include Pennsylvania bluestone in your hardscaping projects.
In this blog post, we’ll take a closer look at the history, use and benefits of this outstanding stone.
Bluestone is a fascinating and complicated natural stone native to Pennsylvania (for whom the material is named), New Jersey and parts of New York. Bluestone has many options to consider in designing and installing your outdoor living project. But, if you have decided on the uniquely beautiful bluestone, you must also deal with the issues of quality and availability.
Quality: The primary quality consideration occurs in natural clef flagstone, both pattern and irregular. Because the appealing unique surface of natural clef flagstone is formed by natural forces, the consistency of that surface is unpredictable. Some pieces can exhibit huge variation in thickness, dramatic shifts in surface texture, even natural warping creating a bowed rather than flat piece. Some people find this variation exotic and attractive, some do not. If you are looking for flagstone with the flattest surface, the least variation in surface and thickness, you are looking for the rarest material.
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In earlier articles, we have discussed the basics of Pennsylvania Bluestone flagstone, its shapes, textures and colors. If you have read these articles or even simply reviewed a portfolio of pictures, you have a good feel for this wonderful natural stone. These factors, plus the process of installation are both important to its final appearance. Thickness is a key factor in installation options.
Two primary installation options exist for bluestone flagstone: dry and wet setting. Dry setting means installing over a compacted stone sub-base and a screeded stone dust setting bed with a flexible finishing joint material. Wet setting means installing over a concrete base on a mortar bed with a solid finishing joint material. Examples of flexible finishing joint materials include stone dust (aka screenings), polymeric stone dust, decorative stone, and topsoil. The most typical solid finishing material is mortar.
Natural clef pattern and irregular flagstone are available in two major thicknesses: 1” and 1 ½”. Because the thickness of these products results from natural separation along fault lines, the thickness is neither consistent nor exact. The thickness of one piece will vary across the length and breadth of the piece. If the thickness is generally between ½” and 1 ¼”, the flagstone is considered 1” flagstone. If the thickness is over 1 ¼”, it is generally considered 1 ½”. When flagstone is particularly thin, i.e., ½” or less, it is separated out at ½” flagstone. Some quarries will separate flagstone that is 3” or over into “heavy” flagstone. The extra thin and extra thick material are frequently included in 1” and 1 ½” categories.
For dry setting application, you will need 1 ½” material. This thickness is required to provide enough strength to handle minor shifts in the base and setting bed caused by geological and weather conditions. The wet setting application, you will want 1” material. This thickness has less variation within and between pieces making it easier to compensate for those differences in the mortar base. 
Thermal flagstone is cut to a nearly exact thickness, usually 1” or 1 ½”. For dry setting, use 1 ½”. For wet setting, either thickness is acceptable as there is no variation in thickness to be absorbed in the mortar bed.
If you use 1” bluestone flagstone in a dry set project, you risk having the material crack or break over time. If you are doing the project yourself and wish to avoid the weight of 1 ½” material, you can accept this risk and replace broken pieces later as necessary. If you have a professional contractor installing your project, make sure 1 ½” material is used.
Lilac Irregular PA Bluestone Flagstone Color
Pennsylvania Bluestone, also known as flagstone, comes in a variety of shapes and textures as discussed in an earlier article entitled PA Bluestone Flagstone – Shape & Texture. All of these options also come in multiple colors.
When most people think of flagstone they think blue: Pennsylvania Bluestone. However, most flagstone is actually not blue in color.
The color of flagstone is caused by the minerals in the ground where the flagstone is formed. Most flagstone is full color also called variegated. This mean that a full range of minerals run through the quarry such that each piece from that quarry can be blue, gray, green, brown, lilac, or rust in color. Most pieces will exhibit multiple colors swirled together in unpredictable patterns. Many believe it is the unique combinations of color that gives flagstone its most compelling beauty. Natural clef pattern and natural clef irregular flagstone have the most color variety and drama. In full color thermal flagstone, much of the drama is removed by the flaming process. Multiple colors still exist but appear more subtle. And the rust color is nearly non-existent in thermal. Tumbling also reduces the dramatic lines of color separation, although not quite as much as the thermal process.
“True Blue” Pennsylvania bluestone occurs less often in nature than full color. True blue flagstone comes out of the quarry in a consistently blue-gray color. A range of blue shades can appear in true bluestone, but the variation is very minor. A large blue flagstone patio may exhibit what appears as waves of blue shades similar to looking at water, but the variation will be negligible. Blue flagstone appears the same color in all forms, i.e., natural clef, thermal, and tumbled.
Occasionally, a large section of a bluestone quarry will be deep reddish – purple. The industry calls this color lilac. Lilac flagstone may also have rust color veins but the underlying color is lilac rather than blue.
When choosing color for your Pennsylvania Bluestone project, be sure to see samples. Samples will give you an idea of the range of variety and the core colors of the various options. However, remember, flagstone is a natural stone. Variety is inherent in the product and in the attraction of natural stone. The material you purchase and its installation will be unique. If you are looking for color consistency, you should consider a manufactured, simulated flagstone product.
If you are considering PA bluestone flagstone as the material for your patio or other outdoor living project, you have several additional decisions to make: Shape, texture, color, thickness, and quality. This article discusses the options for shape and texture. The range of shape and texture options is far greater than you might expect.
Two basic shapes are available: irregular and pattern. A piece of irregular flagstone is generally two to three feet wide by three to four feet long with a completely random amoeba-like shape. Irregular flagstone is packaged in a pallet vertically or standing up which is why it is often called “stand-up” flagstone. Pattern flagstone is cut at the quarry into squares and rectangles in a large selection of sizes. Standard sizes (in inches) are 12×12 up to 24×36 in six inch increments (i.e., 12×12, 12×18, 12×24, 12×30, 12×36, 18×18, 18×24, 18×30, 18×36, 24×24, 24×30, 24×36). Because of the range of measured sizes, pattern is also referred to as “dimensional” flagstone. Larger pattern sizes are less readily available. Smaller patterns sizes are re-cut from broken pattern and used for tumbling only.
Three textures characterize flagstone: Natural clef, thermal, and tumbled. Natural clef is the irregular somewhat wavy surface that is created by nature when flagstone separates or splits along natural fault lines. Thermal refers to the effect created when flagstone is cut and then flamed to remove any irregularities. Thermal is completely flat but has a slightly bubbly surface (almost like a manufactured non-slip finish) created by the flaming process. Tumbled flagstone is literally tumbled in a machine similar to a large drier. The tumbling process smooths the top and bottom of the flagstone and breaks off sharp edges leaving a soft rounded edge.
Both irregular and pattern are available natural clef, thermal, and tumbled. Perhaps obviously, these combinations of shape and texture result in a huge range of options. And, options can be combined, e.g., natural clef pattern patio with irregular natural clef sidewalk. Be sure to see samples of the textures and shapes of PA Bluestone Flagstone you are considering before making your final decision.
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.