Exposure

The rock visible at the Earth's surface, in a cliff, quarry, cutting or at the sea shore.

Hand specimen

A fist-sized piece of rock.

Close-up

Magnified view of the surface of a piece of rock.

Thin section

A thin slice of rock seen under the microscope by transmitted light.

Thin section under plane polars and crossed polars (x-polars)

A thin section of a rock is a very thin slice (0.03mm thick) of the rock stuck on to a glass microscope slide; light can pass through rock slices of this thickness so that the details can be seen under a microscope.

The microscopes that geologists use to study thin sections of rock include two polarizing filters. The polarizing directions of the two filters are at right angles to each other. The lower polar is normally used and thin sections are viewed in "plane-polarized light" (light waves on one plane only). In plane-polarised light, minerals exhibit their "true" colour.

The upper polar (also known as an analyser) can be moved in and out of the path of light. When the upper polar is introduced into the path of light, the colour that is observed is an "interference colour" and not true colour. This is known as viewing the thin section under "crossed polars".

Rock in use

The rock in use in the built environment, within an industrial process or in gravestones, sculptures, ornaments, etc.