A few minerals sometimes show a kind of false [cleavage] known as [parting]. Parting, unlike cleavage, is not constant and does not occur in every specimen of a particular mineral. For this reason, it is not a very dependable means of identification.
[Fracture]
Minerals also break in another way. When the break is in a different direction from that of the [cleavage] or [parting], it is known as the [fracture]. A fracture is called [conchoidal] if the mineral’s broken surface is curved like the inside of a spoon or shell. Thick pieces of glass break with this conchoidal fracture. A fracture is described as hackly if the broken surface has sharp, jagged edges; as even, if the surface is generally flat; and as uneven, if it is rough and not flat. If the mineral breaks into splinters, its fracture is called splintery.
[Specific Gravity]
The [specific gravity] is a measure of whether a mineral is heavy or light. It is a comparison of the weight of a piece of the mineral with the weight of an equal volume of water. The mineral [quartz], for example, has a specific gravity of 2.65. This means that a piece of quartz is a little more than 2½ times as heavy as an equal volume of water. Accurate measurements of specific gravity can be made in a laboratory. You can, however, learn to estimate specific gravities just by lifting various minerals and judging whether they are heavy or light.
Effervescence in Acid
This is a property that depends on the chemical composition of the mineral. Carbonate minerals, which contain (in addition to at least one other [element]) three parts of oxygen and one part of carbon, can be tested with dilute hydrochloric acid. When a drop or two of this acid is put on a carbonate mineral such as [calcite] (calcium carbonate, CaCO₃), the acid begins to bubble and fizz. The fizzing or effervescence is caused by the carbon dioxide gas that is formed when the acid and mineral come in contact with each other. This test is also helpful in identifying rocks, such as [limestone] and [marble], that contain carbonate minerals.