Figure 43 (a) Olivine 'pebbles' with distinctive olive-green colour (dish is 3 cm across). (b) Plane-polarised light image of olivine in a gabbro. The many irregular cracks containing iron oxide (optically opaque) are characteristic of olivine, as is an absence of cleavage. This olivine has partially altered to a green- or brown-coloured mineral, in this case, serpentine. Where the olivine is not altered, it is normally colourless and exhibits no pleochroism (field of view 2.6 mm across). (c) The same field of view as in (b) between crossed polars, the olivine having characteristic second-order interference colours.