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Gabbro is a coarse-grained durable rock with a crystalline structure. It has many aesthetic and practical uses, e.g. for monuments, gravestones, kitchen bench tops and sea defences.
Igneous: Igneous: Formed from molten magma cooling and solidifying while deep underground (therefore a 'plutonic igneous' rock).
Source location: Tasman Bay. Other locations include Northland, Marlborough and Southland.
Sizes: Select Shelf, Hand or Small specimen from the drop-down menu.
Note: Images are examples of specimens only. Sizes and shapes will vary.
More detail
Structure
It has a tightly interlocking, crystalline structure and is made up mostly of dark minerals like pyroxene and calcium-rich plagioclase feldspar, with little or no quartz.
Type
Plutonic (intrusive) igneous rock – the deep-cooled, coarse-grained version of basalt.
Physical properties of gabbro
- Colour: Usually dark grey to black, sometimes with a greenish tint; can show small lighter flecks from the feldspar, but overall it looks much darker than diorite.
- Lustre: When polished, gabbro has a shiny, glassy look and is often sold as “black granite”.
- Hardness: A very hard, tough rock – about 6–7 on the Mohs hardness scale, similar to quartz.
- Density: Quite heavy and dense, typically around 2.8–3.1 g/cm³, making it heavier than most granites and diorites.
- Fracture: Breaks with a granular, slightly uneven surface across its interlocking crystals.
General properties of gabbro
- Composition: Mostly dark pyroxene minerals and calcium-rich plagioclase feldspar, with small amounts of olivine or hornblende; it has low silica content, so it’s classed as a mafic (dark, dense) rock.
- Texture: Medium to coarse grained – you can usually see the individual crystals with the naked eye because it cooled slowly underground.
- Durability: Very strong and hard-wearing; it resists weathering and physical wear, so it lasts well in buildings and roads.
- Heat Resistance: Handles heat well and doesn’t deform easily at high temperatures.
- Low Porosity: It doesn’t soak up much water, so it’s less prone to damage from freezing and thawing.
Uses of gabbro in New Zealand and worldwide
- Roads and Concrete: Most gabbro is crushed and used as aggregate for concrete, road base, asphalt, and railway ballast because it’s strong and long-lasting.
- Building Stone: Cut into slabs and polished for building facades, floor tiles, wall cladding, steps, curbs, and paving – often marketed as “black granite”.
- Landscaping: Used in gardens and public spaces for paths, steps, retaining walls, cobbles, and decorative features where a dark, hard stone is wanted.
- Monuments and Headstones: Polished gabbro is popular for cemetery markers, headstones, and memorials because of its dark colour and resistance to weathering.
- Sculpture and Interiors: Used for sculptures, kitchen benchtops, and interior design pieces, though its hardness makes it harder to carve than softer stones.
- Industrial Uses: Crushed for drainage stone, fill material, and other industrial applications that need a tough, dense rock.
- Geological Interest: Studied by geologists to understand how mafic magmas form layered intrusions and oceanic crust.