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2 Rock Substance Description
2.1 Rock Type
The number of different rock types that are encountered in coal basin environments is generally small and can usually be reduced to: conglomerate, sandstone, siltstone, mudstone, claystone and coal ± intrusive rocks in the form of dykes or sills.
Sometimes you will encounter entrenched (even if wrong) local terminology. For example, strictly mudstone consists of both clay and silt sized particles, siltstone mainly silt, claystone mainly clay. Shale is characterized by its fissility and compositionally may be a mudstone, siltstone or claystone. However usage is not always so precise.
Be on the look out for tuff, often it will have the appearance of a mudstone or claystone.
2.2 Colour
The colour of the rock should be described in the moist condition using simple terms such as –
black, white, grey, red, brown, orange, yellow, green or blue.
Borderline colours should be described as combinations like ‘red-brown’ not ‘reddish brown’.
Colour intensity may be described as pale (not light), dark or mottled [2].
2.3 Grain size
Grain size refers to the average dimension of the mineral or rock fragments. A classification is given in Table 1.

2.4 Texture and fabric or structure
The texture and fabric of a rock specifically refers to the arrangement of the constituent grains or crystals in a rock. It can provide an indication of how the rock formed. For example:
• In sedimentary rocks bedding indicates depositional conditions
• In igneous rocks texture indicates the rate of cooling
• In metamorphic rocks the foliation indicates stress conditions