A Note on Stone Types
Historically, it has been commercial practice to group stones within performance and behavioral groups as opposed to true scientific definition. This is recognized in several ASTM International standards. While scientifically there are hundreds of rock type identifications, only nine groups are commonly acknowledged commercially: Granite, Limestone, Marble, Onyx, Quartzite, Sandstone, Serpentine, Slate, Soapstone, and Travertine. This means that some rocks are included in groups which are not perfectly coincident with their scientific definition (see table below). The National Building Stone Database uses both commercial and scientific classification types, allowing users to search for a particular stone using either name. 1
|Generic Grouping||Petrographic Name||Common Name||Commerical Group (ASTM)|
Grain size (or crystal size in crystalline rocks) is an important parameter in describing and classifying rocks. The National Building Stone Database has adopted a grain size classification scheme derived from one developed by the British Geological Survey (BGS).2 This scheme is based on the Wentworth (1922) phi scale which is widely used to define grain size in sediments and sedimentary rocks. Modifications incorporated in the BGS scheme extend grain size divisions developed for sedimentary rocks to crystalline rocks, ensuring uniformity between different rock types and simplifying data structure. The grain size divisions for all rock types can now be conveyed in one diagram (see below).
The terms used in the database (i.e. very coarse, coarse, medium, fine, very fine, and cryptocrystalline) correspond to the terms shown in the farthest right column in the chart below. The term cryptocrystalline is used for rocks that are so fine-grained that crystals cannot be readily differentiated under the microscope.3
|Phi units||Clast or crystal size in mm (fractional mm).
|Sedimentary clasts||Size terms||Volcaniclastic fragments||Crystalline rocks, igneous, metamorphic or sedimentary|
|boulders||GRAVEL||blocks and bombs||very-coarse grained
|very-coarse sand||SAND||coarse ash grains||medium-grained
|silt||MUD||fine ash grains||very-fine grained
- 1. J.P. Ingham, "Geomaterials Under the Microscope: A Colour Guide," (London, UK: Manson Publishing, 2011).
- 2. British Geological Survey, BGS Rock Classification Scheme, documentation, available online at http://bgs.ac.uk/bgsrcs/ (accessed 3/20/2015).
- 3. M.R. Gillespie and M.T. Styles, "BGS Rock Classification Scheme: Classification of Igneous Rocks," vol. 1, no. RR 99-06 (Notingham, UK: British Geological Survey, 1999), p. 6., available online at http://bgs.ac.uk/bgsrcs/ (accessed 3/20/2015).