"Typically, you'd have a kit and it would contain a sample of each of the rocks used on the scale and what you'd do is scratch the sample with each until you can't do damage to the rock anymore, and that's how to fit it into the scale. So if one can scratch the rock in question but the next one can't, it will fit in between the two." Senku explained easily enough.

"Unfortunately, I don't have a kit like that, but I've been operating without one for a long time, with some fairly good success. It's just practice. If you do enough in geology, you can tell how hard rocks are with the amount of force you apply to them."