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Dust in the air contains certain small amounts of quartz and other impurities whose hardness is about 7. For this reason, stones of about 7 and under are scratched in wear fairly quickly if worn constantly, and it should be noted that emerald, quartz, and opal are amongst these.

In many minerals, there are certain directions in which a blow with a sharp instrument will cleave a specimen, the parting portions showing smooth surfaces. This tendency to split easily in certain directions with smooth surfaces is known as cleavage, and the cleavage planes are directly connected with crystallographic form,’ the direction being parallel to, or following, a possible face of the crystal. Octahedral cleavage gives four planes of cleavage, cubical gives three planes, prismatic gives two, and pinacoidal or basal cleavage gives one plane of cleavage only.

A stone may be hard, however, and yet be easily cleaved. The most important example of this is the diamond, which possesses cleavage directions carefully followed by the lapidary when fashioning the rough stone in its initial stages. Cleavage is an important property to be considered in the working of many gem stones, but some do not possess this quality at all.

Cleavage should be distinguished from hardness and fracture. Some stones, although comparatively soft (jadeite is an example) are very difficult to work on account of their peculiar internal structure. Fracture, characterized by an irregular surface in distinction to the smooth, flat surfaces of cleavage, may be of help in distinguishing a stone. If it breaks with a concave or convex fracture, showing concentric undulations somewhat resembling the lines of growth on a shell, it is said to be conchoidal. This type of fracture is shown in quartz, obsidian, and glass. Other types of fracture are even, uneven, hackly (when the surface is studded with jagged elevations), and earthy. Most gem stones are, of course, brittle, and they do not possess the flexibility and similar properties which characterize many metals.