Nature of the stone
Bluestone is a fossiliferous crinoid limestone according to the nomenclature of PTV 844 - precision level 5, registered at the IBN on 23 September 2005 - (NBN EN 12670). This compact limestone of grey-blue colour and of upper Tournaisian stratigraphic age also corresponds to the descriptions written in the NIT 220 " La pierre bleue de Belgique dite Petit Granit d'âge géolo- gique tournaisien" and NIT 228 " Pierres naturelles " of the BBRI.
NIT 220 Definition
Belgian blue limestone, also called ‘petit granit’, is a calcite stone with a natural, more or less accentuated bluish-grey colour, characterised by the numerous remnants of crinoids. It is exclusively extracted in Belgium. ‘Petit granit’ is an established name and is appropriate due to the particularly sparkling, shiny aspect of the rock, recalling that of granite. It is preferable to call it ‘blue limestone of Belgium’ to distinguish it from other blue stones with a similar aspect extracted in other countries (mainly Ireland, China and Vietnam).
Bluestone is a compact blue-grey limestone, of sedimentary origin, resulting from the accumulation of innumerable fossilised crinoid articles and fragments cemented into a mass of microcrystalline calcite. The stone also contains other fossils such as shells (including brachiopods), corals and bryozoans. On exposure to the weather, the stone takes on a light grey patina.
The stone to be worked is a bioclastic, crinoid dominant limestone. It is a pack stone, sometimes grain stone (DUNHAM classification) or a biomicrite (FOLK classification). The average composition is 96 to 99% carbonates, 0 to 1% quartz, 0.2 to 0.5% iron salts (mainly sulphides) and 0.2 to 0.4% organic carbon.