What is an ideal building stone?
Over the years many New Zealand stones have been used in buildings, but few have proved both durable and economic to work. Several qualities are needed:
- Ease of working. The best stones to work are moderately soft, lacking cracks or cleavage, and of even grain so that they can be sawn in any direction as well as being carved or shaped.
- Strength and durability. Almost all rocks have high enough strength for most building uses, but only the hardest are suitable for high wear areas (steps, paving). Good building stone also needs to be resistant to decay from frost, acid pollution and general weathering.
- Colour and polish. Pleasing colour and texture is a commercial necessity, and for use in decorative panels a hard dense stone that retains a high polish is required.
Categories of stone
All rocks can be grouped into three major categories:
- Igneous – rocks that have crystallised from magma. Volcanic rocks have erupted from a volcano, while plutonic rocks have cooled at depth, and are often coarse-grained (granite and diorite).
- Sedimentary – rocks laid down as layers, often on the sea floor. This group includes limestone, sandstone and mudstone.
- Metamorphic – either igneous or sedimentary rocks that have been recrystallised by the effects of pressure and heat. Marble is metamorphosed limestone, schist is metamorphosed sandstone, and slate is metamorphosed mudstone.
The technical nomenclature of rocks is complex, and is confused by the fact that most of the common building stones in New Zealand have been given trade names. These combine the quarry location and a generalised rock name that is not always accurate or helpful – for example Coromandel granite (which is diorite rather than granite), Ōamaru stone (limestone), and Hinuera stone (ignimbrite). Overseas building stones are often named for their appearance, such as Red Dragon and Blue Pearl.
Greywacke – a difficult rock
Mountain ranges of greywacke form the backbone of New Zealand, and it is the most abundant rock type in the country. But it is difficult to use as a building stone because the more massive rocks do not split naturally, and in other places it is too fractured. River-worn boulders have been used for building stone walls in some places, and a small number of churches and other buildings have been made of greywacke.