Some people make their own hardness testing kit using a few common objects. The contents of their kit often include:

their fingernail  a copper coin  a nail  a piece of glass  a knife blade  a steel file  a streak plate  a piece of quartz

This field hardness testing kit sounds like a great idea. These items cost very little, and you can pull them out of a belt pack and test the hardness of an unknown specimen in under a minute.

But, before you make your own kit, we want you to be aware of something. We checked the hardness values of “common field kit” items suggested in three introductory geology textbooks and found some of them to be highly variable. The variabilities that we found are shown in the accompanying table.

So, if you are going to use common items for hardness determination, it is highly recommended that you confirm the hardnesses of all items in your kit.

In summary,

your fingernail probably has a hardness between 2 and 2.5  get a real piece of copper (pennies are now made of zinc)  nails are made of many types of metal of various hardnesses  the hardness of glass varies between 4 and 7  knife blades vary between 5 and 6.5  don’t assume that every file is made of steel  we tested streak plates sold by several vendors and their hardness ranged between 5 and 7  a quartz crystal has a reliable hardness of 7

Test the hardness of your common items if you want to get good results when you use them.

Mohs Hardness of Common Objects
Fingernail	2 to 2.5
Copper sheet	3
Nail	4 to 7
Glass	4 to 7
Knife blade	5 to 6.5
Steel file	5 to 6.5
Streak plate	5 to 7
Quartz crystal	7