Geologists use a variety of objects to provide a scale for the rocks or structures in a photograph, such as coins or other common objects which come to hand. Probably the best is a purpose-made graduated scale. In many of my photographs, I use a scale card from the Indiana Geological Survey, which has scales in both inches and centimetres.
I have no connection with the Indiana Geological Survey. I have never even been to Indiana! However, a few years back I participated in a Geological Society of America (GSA) Penrose meeting on Foreland Basins in Spain, and two of the convenors were from Indiana University. The plastic scale cards they provided have proved to be pretty durable. Similar cards are produced by other organizations, such as the GSA .
The photograph caption should always state the size of the scale, e.g.:
|Wavy bedding in the Scarborough Formation. Hammer handle is 200 mm long||Basalt with plagioclase phenocrysts. Coin is 17 mm in diameter.||Sharp-based sandstone-filled fluvial channel overyling floodplain sediments. Student is approximately 1.6 m tall.|
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