Black is beautiful
The boss's bellowing request "get me some plain black granite" may sound reasonable on the telephone. After all, there are few plain uniformly black limestones fit for an exterior facade; uniform black slate has not the strength specified; both tend not to be colourfast and need exacting fading tests because of their essentially carbonaceous or bituminous pigmentation.
The request, in meantime turned into the demand "blacker than black", triggers off an odyssey encompassing quarries, suppliers, stockyard, testing labs and the like, and modest samples generate into elaborate and time-taking mock-ups.
Not exactly, the scenario for a small order of random cut black granite slabs or tiles. When the order involves monumental structures, with large size slabs, all to be faultlessly black, exposed on four sides to climatic variations, then the search gains momentum. The more so with a delivery time limit for a project, and an anniversary deadline thrown into the bargain.
What is meant by "black granite"? To quote from the discussion in ASTM C119-95b Standard we learn the following (please take a deep breath!)
"Black Granites: dark-coloured igneous rocks defined by geologists as basalt, diabase, gabbro, diorite and anorthosite are quarried as building stone, building facings, monuments, and speciality purposes and sold as black granite. The chemical and mineralogical compositions of such rocks are quite different from those of true granites, but black granites nevertheless may be satisfactorily used for some of the same purposes as commercial granite. They possess an interlocking crystalline texture but, unlike granites, they contain little or no quartz or alkali feldspar. Instead, black granites are composed dominantly of intermediate to calcic plagioclase accompanied by one or more common dark rock-forming minerals such as pyroxenes, hornblende, and biotite. Such rocks, because of their relatively high content of iron and magnesium, are designated as ferromagnesian or mafic. An exception is anorthosite which, though commonly dark, consists mostly or entirely of calcic plagioclase". So far a statement by consensus. We can now go ahead to use Black Granite as an umbrella term while offering the material under the (geologically correct) names in italics above.
What is in these names is not always clear. Basalt is actually mineralogically a fine-grained gabbro, but is seldom defined as such. The buyer looking for a gabbro is discouraged when the seller is actually offering unwittingly basalt and the buyer moves on to the next supplier. In the same vain Norite is not mentioned in the Standard but is a gabbro with more orthopyroxene than clinopyroxene. This mineralogical surplus may sound natural to the geologist, but makes correct use of scientific names difficult in commercial practice.
The information (in italics) in the Standard is that the mafic iron containing minerals gives the stone its black colour. More specifically the magnetite needles are supplying the deep black in Belfast Black (see below), help the stone consultant in evaluation -all the boss wants is blacker than black.
Until recently, the ASTM discussion was considered informative rather than of normative interest. However with the 'enforcement' of the CEN stone standards with mandatory petrographic description requirements as starting point, these have to be taken seriously. They affect specification, which govern price and quality, both serious items in the industry, besides legal coverage for the buyer and seller.
This is especially true for plain black granites where the grain sizes of the matrix are to be uniform throughout the deposit, causing more often than not the quarry recovery rates (the percentage of marketable material obtained from any volume of material excavated) of less than 5% of the prime material, to the regret of the quarrier. Ideally the grain size of the major constituent,plagioclase, for a fine-grained material should average less than 0.5 mm.
At the end of the slab fabrication, the colour may range from pitch-black (whatever that implies) to darkish grey. The polished or honed stone may have specks, some causing a glint like in Galaxy Black, tiny white spots in any size, shape, colour or shade; may have a brownish, bluish, greenish hue and may be described as deep, dark, medium, light or greyish. Often a gradual transition from the blacks into the greys takes place at the quarry, calling for selective quarrying to reduce colour variation and increasing extraction costs. Samples for inspection are to be polished on one side and honed on the obverse, even if honed is required, as the true and more lasting colours are brought up by polishing, besides showing any defaults.
With the advent of denomination criteria in the approved CEN Standards the discussion of scientific versus commercial denomination has become hopefully redundant. Just for old times' sake let us look over some denominations of “Black Granites” (or dark greys with an essentially with black matrix). Some of the original names are still being used to indicate varieties. The names in Italics may have become the main appellation, especially by old-timers in the industry.
The classification into categories depend on the petrographical examination (CEN 12407) Standard of random samplings from quarry faces and locations, that may have been changed with the years, and therefore may have turned arbitrary. The categories may even be different for stone within the same working. Anyway, the list is indicative, not exhaustive. Application of CEN 12407 may in good time rectify the situation and a name will be the proper name. Some of the better known industrial varieties include:
Belfast Black - Nero Assoluto; SSY Africa; Marlin; Nero Africa (S.A)
Rustenburg Grey (S.A]
Bertania - Noir Royal,Rustenburg Black; Starlight (S.A.)
Impala - Jasberg; Rustenburg
Saari Black (Finland)
Brazil Black- Black Ceara
Ebony Black-Swedish Black ;SS
Svart EGC; Black EGC (Sweden)
Kuru Black- Kuru Black Star; Tempere (Finland)
Negro Ochavo-Negro Valencia (Spain)
Indian Black-KH Black( India)
Salem Blue (India)
Star Galaxy -Black Galaxy; Galaxy Gold (India)
Zimbabwe Black -Nero Zimbabwe; Belfast Zimbabwe (Africa)
Nero Fine;Nero Coarse;Dark Grey Zimbabwe
Transkei Black (Transkei)
Kalopa Black (Malawi)
Omenje Dolerite (Namibia)
(Gabbro) - Anorthosite)
Angola Black;Palm Black;Egal Black
The various names in Italics may be typically used in a particular country as e.g. Jasberg and Bertania in Belgium. Each name has some kind of history or anecdote behind it. Bon Accord was named after the Town Square near the block landing point in Aberdeen. When searching for a source of rocks not commonly or widely used in large quantities, like Black Granite, a look at traditional records and applications may help in the final decision, in addition to the recommendations of the supplier.