STREAK

When a softer mineral is rubbed or scratched across a harder surface, a fine layer of the softer mineral may be grinded and deposited as a fine dust or a coloured mark referred to as a 'streak'. For example, the writing left on paper when a graphite pencil is used.

Some minerals leave a characteristic streak, for example the dark blood-red streak of haematite and the grey streak of galena. Testing for streak is destructive and is not generally undertaken by gemologists, particularly as it is of little use with small, cut stones. However, it may be useful with larger uncut specimens, rock fragments and decora­tive pieces, as well as to check for dying or colour impregnation. For example, malachite has a green streak, but another rock type dyed green to imitate malachite may have a white streak and only a thin green dyed layer.