Twinning
Another modification of the simple forms which will be met occasionally is twinning. By this is meant two crystals growing together as though placed side by side on some one of the faces, and then revolved until the two axes which would normally be parallel are at some definite angle with each other, 60°, or 180° which is commoner. The surface of contact between the two crystals is called the composition face, and as no more material can be added on that face the crystals continue to grow developing the other faces, and we find faces in contact with each other which should be at the opposite end or other side of the crystals. This contact of faces which should not come in contact, and the presence of reentrant angles are indications of twinning. In some minerals the twinning may be repeated time and again, and if the twinning is on one of the end faces a branching structure results, as in frost and snow crystals, or the multiple twinning may be of crystals growing side by side when the final form will approximate a series of thin sheets placed side by side as in some feldspars. The peculiar forms characteristic of individual minerals are taken up under the respective minerals.
Other important properties of minerals are hardness, cleavage, specific gravity, streak, luster, and color.