Clinker

i. According to Grapes (2006) Clinker is a hard, fused and/or partially melted sedimentary rock, with well preserved shaley texture, forming in burnt coal seams or bituminous sediments. More glassy, vesicular molten rock, that is sometimes incorrectly named scoria, should be called buchite. Characteristic features of clinker include a resemblance to pavement brick (klinkaerd) and a "metallic" sound when hit by a hammer. Clinker is the most common rocktype found in burning coal mining dumps and is usually red or pink, less commonly yellow or white in colour. It is usually composed of quartz, feldspars, mica and minor mulliteAl_4+2xSi_2-2xO_10-x.

White clinker-like rocks are sometimes called porcellanites, though this can have widely varying definitions.

Clinkers are often found fused together from various lithologies to form a pyrometamorphic breccia. They may also occur as xenoliths within recrystallised rocks called parabasalts.
Ref: ACSG, 2

ii. Coal that has been altered by an igneous intrusion. Compare with: core and natural coke

iii. Partially fused intermediate product, formed from limestone and claystones, used in the manufacture of portland cement.

iv. Fused or partly fused coal ash, a byproduct of combustion.