40 . PHYSICAL PROPERTIES OF MINERALS. 
5. DIAPHANEITY, LUSTRE, COLOR. 
1. DIAPHANEITY. 
Diaphaneity is the property which many objects possess of 
transmitting light ; or, in other words, of permitting more or 
less ight to pass through them. This property is often called 
transparency, but transparency is properly one of the degrees 
of diaphaneity. The following terms are used to express the 
different degrees of this property : 
Transparent—a mineral is said to be transparent when the 
outlines of objects, viewed through it, are distinct. Hxample, 
glass, crystals of quartz. 
Subtransparent, or semitransparent—when objects are seen 
but their outlines are indistinct. 
Translucent—when light is transmitted, but objects are not 
seen. Loaf sugar is a good example; also Carrara marble. 
Subtranslucent—when merely the edges transmit light faintly. 
When no light is transmitted the mineral is described as 
opaque. 
2. LUSTRE. 
The lustre of minerals depends on the nature of their surfaces, 
which causes more or less light to be reflected. There are dif- 
ferent degrees of intensity of lustre, and also different kinds of 
lustre. 
a. The kinds of lustre are six, and are named from some 
familiar object or class of objects. | 
1. Metallic—the usual lustre of metals. Imperfect metallic 
lustre is expressed by the term submetallic. 
2. Vitreous—the lustre of broken glass. An imperfect 
vitreous lustre is termed subvitreous. Both the vitreous and 
subvitreous lustres are common. Quartz possesses the former 
in an eminent degree; calcareous spar often the latter. This 
kind of lustre may be exhibited by minerals of any color. 
3. Resinous—lustre of the yellow resins. Hxample, some 
opal, zinc blende. 
4. Pearly—iike pearl. Example, talc, native magnesia, stil- 
bite, etc. When united with submetallic lustre the term 
metallic-pearly 1s applied. 
5. Silky—like silk; it is the result of a fibrous structure. 
