Chalk rock (rounded)

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Chalk is a soft, white, rock composed mainly of calcium carbonate minerals. Specimens are rounded by physical weathering. It is a specific type of limestone characterised by its softness and the presence of finely grained, microscopically sized shells.

The smooth surfaces make rounded rocks especially suitable for scratch tests to determine hardness.

Sedimentary: Formed from the accumulation of microscopic marine fossils such as foraminifera and coccoliths (therefore technically an 'organic sedimentary' rock).

Source location: Marlborough.

Sizes: Choose from dropdown menu either Shelf specimen (300–800g), or Hand specimen (150-300g), or Small specimen (less than 150g).

Note: Images are examples of specimens only. Sizes and shapes will vary.

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Chalk is a soft, white, sedimentary rock that is formed from the accumulation of microscopic marine fossils such as foraminifera and coccoliths. It is composed mainly of calcium carbonate minerals.


Organic sedimentary.

Physical Properties of chalk

  • Colour: Typically white, but can also be grey, yellow, or brown.
  • Lustre: Dull.
  • Texture: A fine-grained texture and is soft to the touch.
  • Hardness: 1 to 2 on the Mohs scale, making it very soft.
  • Density: Ranges from 1.5 to 2.7 grams per cubic centimetre.
  • Porosity: Porous, with a porosity ranging from 30% to 60%.

General Properties of chalk

  • Solubility: Chalk is soluble in acidic water, which can result in the formation of karst landscapes such as caves and sinkholes.
  • Electrical Insulation: Chalk is a good electrical insulator due to its low conductivity.
  • Chemical Reactivity: Chalk is reactive with acids e.g. white vinegar, and can be used as a source of carbon dioxide gas.
  • Carbonate Buffering: Chalk has the ability to neutralise acid, which makes it useful in environmental applications such as water treatment and soil stabilisation.

Uses of chalk in New Zealand and worldwide

  • Education: Used as a writing tool on blackboards and slate boards.
  • Construction: Used as a construction material, particularly in the form of crushed stone and aggregates for roads, concrete, and buildings.
  • Agriculture: Used in agricultural applications to neutralise acidic soils and provide essential nutrients such as calcium and magnesium.
  • Chemical Production: Used in the production of chemicals such as calcium oxide, calcium hydroxide, and sodium carbonate.
  • Water Treatment: Used in water treatment to neutralise acidic water and remove impurities such as heavy metals.
  • Art: Used in art as a drawing medium, particularly for pastel drawings.
  • Personal Care: Used in personal care products such as toothpaste and cosmetics.
  • Geological Studies: Studied by geologists to understand the processes of sedimentation and the history of the earth's crust.