BASALT SCORIA: Volcanic, generally dark grey, fine-grained (aphanitic) but may be porphyritic phenocrysts may be olivine, pyroxene, feldspar.  Textural varieties include Vesicular basalt, and Scoria (highly vesicular 

basalt). Silica content (SiO2) approx. 47-53%.  Weathered surfaces often mottled brown because of 

relatively high iron content.  Uses – aggregate, fill etc. in construction and roading (best if olivine content is 

low); armour rock for seawalls; dimension stone, e.g. stone walls, curb stones, paving stones etc.; very 

porous basalt (scoria) can be used to improve sub-surface drainage, as trench backfill, in manufacture of 

lightweight blocks etc. 

ANDESITE: Volcanic, colour variable but generally medium grey (lighter coloured than basalt), porphyritic 

texture typical, phenocrysts generally feldspar, pyroxene, sometimes olivine, hornblende.  Silica content 

(SiO2) approx. 53-63%.  Uses – aggregate, fill etc. in construction and roading (often not ideal as concrete 

aggregate because of high silica content). 

VESICULAR BASALT: Volcanic, generally dark grey, fine-grained (aphanitic) but may be porphyritic, phenocrysts may be 

olivine, pyroxene, feldspar.  Textural varieties include: Vesicular basalt, and Scoria (highly vesicular 

basalt). Silica content (SiO2) approx. 47-53%.  Weathered surfaces often mottled brown because of 

relatively high iron content.  Uses – aggregate, fill etc. in construction and roading (best if olivine content is 

low); armour rock for seawalls; dimension stone, e.g. stone walls, curb stones, paving stones etc.; very 

porous basalt (scoria) can be used to improve sub-surface drainage, as trench backfill, in manufacture of 

lightweight blocks etc. 

DACITE: Volcanic, colour variable but medium grey common, generally porphyritic texture, phenocrysts 

generally feldspar, hornblende and often quartz.  Silica content (SiO2) approx. 63-70%.  Uses – aggregate, 

fill etc. in construction and roading (often not ideal as concrete aggregate because of high silica content). 

RHYOLITE OBSIDIAN: Volcanic, colour variable but generally light-coloured, usually porphyritic texture, phenocrysts 

generally feldspar and quartz, often with hornblende, mica.  Other varieties of rhyolite include: obsidian 

(volcanic glass - glassy, generally black or dark green) and pumice (light coloured, very vesicular (full of 

holes) and not very dense).  Ignimbrite – formed by the lithification of ash fall or pyroclastic flow deposits. 

Very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice, commonly with 

scattered rock fragments.  Rhyolite silica content (SiO2) approx. 70-78%.  Uses – aggregate, fill etc. in 

construction and roading (often not ideal as concrete aggregate because of high silica content); obsidian can 

be carved into jewellery;  pumice is used as an abrasive (especially in the cosmetic industry), and can also 

be incorporated into lightweight building materials. 

RHYOLITE PUMICE: Volcanic, colour variable but generally light-coloured, usually porphyritic texture, phenocrysts 

generally feldspar and quartz, often with hornblende, mica.  Other varieties of rhyolite include: obsidian 

(volcanic glass - glassy, generally black or dark green) and pumice (light coloured, very vesicular (full of 

holes) and not very dense).  Ignimbrite – formed by the lithification of ash fall or pyroclastic flow deposits. 

Very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice, commonly with 

scattered rock fragments.  Rhyolite silica content (SiO2) approx. 70-78%.  Uses – aggregate, fill etc. in 

construction and roading (often not ideal as concrete aggregate because of high silica content); obsidian can 

be carved into jewellery;  pumice is used as an abrasive (especially in the cosmetic industry), and can also 

be incorporated into lightweight building materials. 

RHYOLITE IGNIMBRITE: Volcanic, colour variable but generally light-coloured, usually porphyritic texture, phenocrysts 

generally feldspar and quartz, often with hornblende, mica.  Other varieties of rhyolite include: obsidian 

(volcanic glass - glassy, generally black or dark green) and pumice (light coloured, very vesicular (full of 

holes) and not very dense).  Ignimbrite – formed by the lithification of ash fall or pyroclastic flow deposits. 

Very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice, commonly with 

scattered rock fragments.  Rhyolite silica content (SiO2) approx. 70-78%.  Uses – aggregate, fill etc. in 

construction and roading (often not ideal as concrete aggregate because of high silica content); obsidian can 

be carved into jewellery;  pumice is used as an abrasive (especially in the cosmetic industry), and can also 

be incorporated into lightweight building materials. 

GABBRO: Plutonic, colour very dark (black typical), dense, phaneritic texture (medium to coarse-grained), 

crystals predominantly feldspar with pyroxene and olivine; chemical equivalent of basalt.  Uses –  

aggregate, fill etc. in construction and roading; dimension stone (called black granite) for building facings, 

foyers etc. 

DIORITE: Plutonic, typically speckled black and white, phaneritic texture (medium to coarse-grained), crystals 

feldspar, hornblende, quartz;  chemical equivalent of andesite and dacite.  Uses – aggregate, fill etc. in 

construction and roading (often not ideal as concrete aggregate because of high silica content); dimension 

stone for building facings, foyers etc. 

GRANITE: Plutonic, colour variable but often pinkish-white, phaneritic texture (medium to coarse-grained), 

crystals interlocking feldspar and quartz often with smaller amounts of mica and hornblende;  chemical 

equivalent of rhyolite.  Uses – aggregate, fill etc. in construction and roading (often not ideal as concrete 

aggregate because of high silica content); dimension stone for building facings, foyers etc; bench tops, 

counters, monuments, headstones, statues.