While we have discussed previously various types of stone available to the builder, it is worth pointing out that even within one type of stone there is considerable variance. Sometimes, even within various types of quality building stone, there will be poor-quality specimens that are not ideal for use in building.
Crumbly stones are primarily a problem with sedimentary rocks. Often, a stratum of poor-quality rock will be found between layers of good rock. If you have ever pried layers of limestone out of the ground, you will likely find a thin layer of pasty clay between the rock you pried up and the next layer of rock below. This clay is, in fact, decayed rock. The decayed rock does not have to be in the form of clay. It is possible to find an apparently solid layer of rock which, upon removal, crumbles into fragments. In such a strata of rock, one may still find sizable pieces that hold together, but the quality of such stone is suspect. Broadly, if a rock tends to break up into numerous thin layers, the rock probably will rapidly disintegrate in freeze/thaw cycles.
Based on observation, crumbly stones are much more likely to hold up in a mortarless structure than they will in any kind of mortared work. On the whole, the best use of such a rock is to be broken up into fragments, which pieces can then be used as shims. Some crumbly rocks, however, won’t even fragment; they will rapidly turn into sludge. A good way to test the quality of a suspect rock is to clean the surfaces of a sample thoroughly, then stir it around in a water bath. If the water in the water bath rapidly becomes highly discolored, the rock will likely disintegrate over time. Usually such testing is not required, as it often becomes quickly obvious if a rock is too soft to be used in a build.
Stones may not be crumbly but still have weak spots. These weak spots are caused by small seams and/or hollows in the stone. They may not be apparent from the outside, but still can cause the stone to break under pressure. Based on personal experience, a stone will tend to split along a major weak line during stonecutting. While this usually results in the stone taking on the wrong shape, it does expose the weakness. Often, seams in the rock can be spotted with close observation.
These weak spots are worth being aware of, but serious weaknesses usually will become apparent before the stone is placed in a structure. It is worth mentioning that a stone can usually be readily split along a visible seam.