This file and its description are from Wikimedia Commons.
Description: Perlite (volcanic glass with white powder appearance). Perlite is a volcanic glass that has many industrial applications, because of its light density and insulating properties.
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).