When a mineral is entirely devoid of crystalline structure, both externally and internally, it is said to be _amorphous_.
Perfect and distinct crystals are the rare exception, most mineral specimens being simply aggregates of imperfect crystals. In such cases, and when the mineral is amorphous, the _structure_ of the _mass_ may be:—
Granular.—When the grains or crystalline particles are invisible to the naked eye the mineral is called _impalpable_, _compact_, or _massive_.
And the _external form_ of the mass may be:—
Stalactitic, forming stalactites or pendant columns.
Amygdaloidal or Concretionary, forming separate globular masses in the enclosing rock.
3. _Hardness._—By the hardness of a mineral we mean the resistance which it offers to abrasion. But hardness is a purely relative term, calcite, for example, being hard compared with talc, but very soft compared with quartz. Hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others, and known as the _scale of hardness_. These are arranged at nearly equal intervals all the way from the softest mineral to the hardest, as follows:—
(1) Talc. (2) Gypsum. (3) Calcite. (4) Fluorite. (5) Apatite. (6) Orthoclase. (7) Quartz. (8) Topaz or Beryl. (9) Corundum. (10) Diamond.
If a mineral scratches calcite and is scratched by fluorite, we say its hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is scratched by orthoclase, its hardness is 6; and so on. There are very few minerals harder than quartz, and hence the first seven members of the scale are sufficient for all ordinary purposes; and these are all included in the series of specimens accompanying this Guide.
Although it is desirable to be acquainted with the scale of hardness, and to understand how to use it, still the student will learn, after a little practice, that almost as good results may be obtained much more conveniently by the use of his thumb-nail and a good knife-blade or file. Talc and gypsum are easily scratched with the nail; calcite and fluorite yield easily to the knife or file, apatite with more difficulty; while orthoclase is near the limit of the hardness of ordinary steel, and quartz is entirely beyond it.