There are 3 types of readily available stone for sculptural use in New Zealand, Andesite, Hinuera, and Oamaru. Andesite is a volcanic very hard stone similar to granite, Hinuera is a sandstone, and Oamaru a limestone. I will cover these 3 separately as to their properties, care and maintenance when used as outdoor sculpture.
ANDESITE: A very hard, volcanic, silicone based stone. Quarries are in the upper North Island and in New Plymouth. Stone color ranges from black through reds, greys, browns, and even dark green. This stone can be taken to a high polish and requires no treatment to sustain its color when placed outside. Due to its density and hardness, this stone can only be worked with diamond power tools. This will cause the cost of a sculpture made from this stone to be appropriately higher. A sculpture made from this material has the same qualities and durability as granite and may be placed in any situation, including water features and will last indefinitely.
HINUERA: A volcanic "tuff" with a 95% silicone content. Hinuera sandstone is a highly durable and attractive stone for outdoor use. First quarried from the Waikato region in New Zealand, it was used in construction since the 1800's. Today, all stonework used in buildings are still in their original condition. The colors range from cream to pink, light brown and grey, and are varied in their appearance. While one stone may be completely cream, another may display "grain" markings similar to those found in wood, making beautiful patterns on the contours of a sculpture. There are 3 types of Hinuera, ranging from soft to hard. The hardest quality ha a "speckled" appearance, with dots of varying colors rather than stripes of "grain".
To Maintain the original color of the stone, a water based silicon sealant is recommended. This can be brushed on, and once applied, sinks to a depth of around 4 inches into the surface of the stone, making absorption of water into the stone impossible. One coating of such a sealant will be adequate to maintain color for up to 7 years. If untreated, the stone will not display any signs of decay, but will grow lichen and moss on the surface, changing the color.
OAMARU: The white, easily sawn "Oamaru stone" is known to geologists as Totara limestone. It is formed of sand sized bryozoan fragments, with vast numbers of microfossils from shells and sometimes penguin or whale bones and sharks teeth. It is mainly used as a building stone, its excellence in this capacity lying in its fine-grained, even texture, and freedom from joints and impurities. Nowadays, limestone from parkside quarries in the South Island of New Zealand is use in buildings in the North and South Islands, and has been exported to Australia and Japan. While it is a highly durable outdoor material, Oamaru stone requires maintenance, cleaning and sealing for the stone to retain its natural white-creamy texture and color. When it is dry, it should be treated with a top quality water repellent silicone. In normal circumstances, correctly treated surfaces remain water resistant for up to 7-10 years. While it is highly durable in outside situations, it is also a fairly soft stone and care should be taken when transporting sculptures of this material to ensure breakage or damage does not occur.