Thus far our discussion has centered about crystals as individuals, but, in most cases by far, they form groups or aggregates. Most commonly crystal grouping is very irregular, but by no means rare is parallel grouping where whole crystals, or more usually parts of crystals, have all corresponding parts exactly parallel. But most remarkable of all are the twin crystals in which two or more crystals intergrown or in contact have all corresponding parts in exactly reverse order. The conditioning circumstances under which twin crystals develop are unknown. In the light of the facts and principles above explained, the reader will more than likely agree with the author that crystals rank very high among nature's most wonderful objects. But there are still other characteristic features of crystals naturally resulting from their marvelous structure. Some of these will now be briefly referred to. [Illustration: Fig. 73.--Figures illustrating twin crystals: a, gypsum (Monoclinic system); b, fluorite (Isometric system); c, cassiterite (Tetragonal system). (After New York State Museum Bulletin.)] Many crystals and crystalline substances exhibit the important property known as cleavage which is the marked tendency to break easily in certain directions yielding more or less smooth plane surfaces. As would be expected, a cleavage surface is always parallel to an actual, or at least a possible, crystal face, and it takes place along the surfaces of weaker molecular cohesion. The degree of cleavage varies from almost perfect, as in mica, to very poor or none at all, as in quartz. The number of cleavage directions exhibited by common minerals is illustrated as follows: mica, one; feldspar, two; calcite, three; and fluorite, four. It is a striking fact that when a crystal or cleavage piece is placed in a solvent, the action proceeds with different velocities in crystallographically different directions and little pits or cavities, called etching figures, are developed on some or all of the faces. Since the symmetry of these etching figures and their arrangement upon the faces are directly related to, and natural effects of the crystal symmetry, the figures often furnish an important method of placing a doubtful crystal or even merely a cleavage fragment in its proper symmetry class.