Lustre. 
This property depends upon the nature of the surface of a substance, which 
causes more or less light to be reflected. The degree in which minerals possess it 
are classed under six heads :— 
1. Metallic : When a substance has the appearance of a metal. 
2. Vitreous : When a substance has the appearance of broken glass. 
3. Resinous : When it looks like resin. 
4. Pearly : When it looks like pearl. 
5. Silky : When it looks like silk. 
6. Adamantine: When it looks like a diamond. Brilliant or shining. 
Electricity and Magnetism. 
Many minerals become electric on being rubbed, when they will attract cotton 
or any other light substance, whilst one of the iron ores, called lodestone, is naturally 
magnetic, and several of the others which contain iron are attracted by the 
magnet. 
Taste and Odour. 
These properties are also possessed by many minerals, and by them they may 
be easily distinguished. 
Cleavage and Fracture. 
Cleavage .—When a mineral breaks up into definite forms, with smooth regular 
faces, such faces are said to be the cleavage planes. 
Fracture .—When a mineral breaks up irregularly under a blow, the appear¬ 
ance of the fresh face is called its fracture. Thus the fracture is said to be even 
when it forms a plane of some extent, uneven when the surface is rough, conclioidal 
when the fractured pieces are concave on one side and convex on the other, 
splintery when the surface presents the appearance of a number of thin-edged 
scales, hackly when covered with numerous sharp points or inequalities. 
Crystallography . — One of the most important branches of mineralogy is 
crystallography, which has of late years developed almost into a separate science, 
but here it is of lit tle practical value in the determination of minerals, as they are 
rarely found in the form of crystals. 
It may be described as a study of the crystalline forms and structure of minerals 
for the convenience of distinguishing species, each mineral being constant in its 
structure and crystalline form. 
The chemical composition always bears a certain relationship to the crystalline 
form. 
The form is almost entirely dependent upon crystallisation, as well as all 
optical characters. 
For this study a knowledge of mathematics is necessary. 
The forms of crystals are very variable, but they may be divided into six 
systems: — 
1. Isometric or regular system (called cubic, octahedral, <fcc.). — In this 
system three axes of equal lengths pass from side to side through a 
point in the centre, cutting each other at right angles. 
2. Dimetric or square prismatic system.—These have three axes at right 
angles to one another, but only two of equal length. 
3. Trimetric or right prismatic system, in which the three axes are of 
unequal length, but cut one another at right angles. 
