English: Elatina Fm diamictite, formed during the Cryogenian Period, adjacent to the Ediacaran GSSP site in Brachina Gorge, Flinders Ranges National Park; outcrop exposed in creek bed. $AUD1 coin for scale.
Using coins for scale is discouraged as it will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins can also reinforce a geographical bias, and some coins' designs are copyrighted.
Ideally, a photograph should include a ruler with the subject (example) or an added scale marking.
SI ("metric") units are the most commonly used worldwide (see meter and centimeter).
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