Of the factors we consider when evaluating rough material for cutting, it’s Integrity is often overlooked by new lapidarists. The qualities that are important to a roughs integrity include aspects such as it’s hardness, it’s toughness, and what I will call it’s degree of separation.
The hardness is simple enough to understand. For our purposes, it is mostly a qualitative aspect that represents the relative ease, or difficulty, in breaking the bonds of a material in order to alter it’s shape. It is important to consider, because there are softer materials, such as apatite, or obsidian (volcanic glass) which are easily scratched, and can easily lose their polish. Others, such as sapphire, are so hard that you just about have to go out of your way to scratch it or alter it’s polish. In this way we consider it’s potential use in jewelry.
When it comes to minerals, there is a quantitative description as well, as we do often refer to minerals by the MOHS Hardness scale, which is numbered. Most minerals, though, will have slight variances in hardness due to malformation. This may be due to impurities, incomplete chemical bonds, or even the differential hardness that is inherent to a crystals structure. It is useful to know this about particular materials, but is briefly mentioned.
A stones toughness is a little more difficult for most people to understand immediately. It is closely related to hardness in the sense that it is reliant on the nature of a materials chemical bonds and structure, and this relationship is likely the cause of any confusion. To put it plainly, toughness is how well a material can withstand impact without separation from itself. The common example is as follows: Consider that diamond has a MOHS hardness of ten, and that it is one of the hardest substances we know of. Now, consider how well a diamond would withstand an impact from a hammer; not well. Diamond is not a tough stone, it is brittle, or rigid, and doesn’t transmit force to the same degree as a well indurated jasper or agate, which can take a beating with less fracture.
The degree of separation I referred to earlier specifically refers to the roughs current and present fracture. If you can see whether or not a material is fractured, and you are familiar with the roughs other aspects you will better be able to decide on the size of your cut, and it’s purpose. This can save the lapidarist a lot of time, and face if you are cutting for someone.
C. Willis Riffe