In geology, it is good practice to take photos of specimens next to measuring tools for scale. On the internet, a banana is a common object for scale in photos, but more precise measurements are critical for science.
This scale is the official photo scale from The Geological Society of America (GSA). It includes centimeter and inch measurements for scalar comparison when photographed on the same plane as rocks and crystals.
The circles function as an evaluation scale for granular material from 1 to 5 millimeters in diameter. The GSA seal can be used for fine focus to ensure everything in the photo is nice and crisp.
As an added feature, the latest geological time scale is printed on the reverse side. This is used by geologists to date various strata.
You can use this card as a scale in any photo where size representation is important, not just rocks. You can even pack it in a hiking bag in case you want to photograph animal tracks or embedded fossils.
This listing is for one time scale card