Crystal is defined to be a definite geometrical form or outline as a result of bounding regularly arranged natural plane surfaces to a mineral or to a prepared chemical substance. Almost all gem minerals are outlined into well-formed crystals. Upon understanding the crystallographic properties of certain minerals, efficient cutting of gems is possible and of great help in the identification of uncut material. A crystal when examined under a geological polarizing microscope may appear to consist one form or have two or more forms. A form is an assemblage of corresponding faces.

Nowadays, the form and the natural plane surfaces of the crystals of minerals are the external or outward expressions of the mineral’s definite internal atomic structures, which are orderly arranged so as to form the crystal. Those that are considered not orderly arranged are called non-crystalline amorphous solids. These solids are found devoid of natural plane surfaces. These crystal formations of minerals may have splendid view under polarized light microscopes for geologists.

Minerals assume different varieties of forms when viewed under petrographic polarizing microscopes. These different forms depend upon the chemical nature of the mineral and the condition of which they are crystallized. There are two interference figures found commonly given by minerals. Minerals may be categorized into two different interference figures, namely, uniaxial and biaxial. Uniaxial means the optical characters of the mineral in question are symmetrical to only one direction, the optic axis. While in biaxial, the mineral has unsymmetrical optical characters to the optic axis, but to the three planes positioned at right angles to each other. Below is an illustration of different crystal systems and habits.

Isometric Crystals – the cross sections exhibited under a reflected light of a polarizing microscope are the common isometric forms such as cubes, octahedron, dodecahedron, and trapezohedron. These have three equal and perpendicular axes. These forms are commonly twinned and are normally considered isotropic. Isotropic means, the light travels through in the same velocity in all directions.

Tetragonal Crystals – cross sections exhibited in reflected light of polarized light microscope are usually the common normal sections of tetragonal prisms of bipyramidal termination or combination of two prisms of two pyramids. The crystals of this system have three axes, which intersect at right angles when viewed in petrographic polarizing microscope. The vertical axis may be longer or shorter than the two equal horizontal axes. Crystals are uniaxial and anisotropic, where light travels through in different direction with different velocities.

Hexagonal Crystals – when viewed under a petrographic polarizing microscope with the upper nicol swung out, this system has two principal types of crystal development, namely, the hexagonal subsystem and the rhombohedral subsystem. The cross sections exhibited by hexagonal subsystem are exhibiting elongated crystal forms, while the cross sections of the rhombohedral subsystem always exhibit rhombic crystal forms and sometimes also hexagonal. The crystal of this system has four axes. The three are equal and are horizontal axes intersecting at 60?. The fourth axis is vertical and is found perpendicular to the other three axes.

Orthorhombic Crystals - crystal formation appear to be symmetrical with respect to the crystallographic axes and apparently become extinct when the axes move to appear parallel to the plane of the vibrating nicol of the petrographic polarizing microscope. The crystal in this system has three perpendicular and unequal axes.

Monoclinic Crystals – crystal formation exhibit inclined extinction. More often the cross sections when viewed under a polarizing light microscope for geologists exhibit symmetrical and sometimes with parallel extinction. The crystals in this system have three unequal axes. Two of which intersect at an oblique angle while the third axis is positioned perpendicular to the other two.

Triclinic Crystals – crystal formation in reflected light of a polarized microscope appear to be of ordinarily inclined extinction with high extinction angles. Often exhibit striking twinning. The crystals in this system actually have three unequal axes, all inclined to each other.

Hardness

Precious stones are those harder than quartz while semi-precious are those that are softer stones. The hardness of a mineral is the measurement of its resistance to scratching or abrasion. There are several methods used to determine the hardness of a mineral. The likes are The Mohs scale and by the use of the hardness pencils or points. Mohs scale is consists of ten minerals arranged in an increasing order of the mineral’s hardness. It is illustrated as follows:
1. Talc 6. Orthoclase
2. Gypsum 7. Quartz
3. Calcite 8. Topaz
4. Fluorite 9. Corundum
5. Apatite 10. Diamond
When two substances will scratch its other equally well, then they appear to be of the same hardness. The fingernail, copper coin, knife blade, piece of glass, or steel can be also used to supplement the Mohs scale.
Fingernail – hardness up to 2.5 Copper coin- hardness up to 3
Knife blade – hardness up to 5.5 Glass – hardness is 5.5
Steel file- hardness range from 6 - 7
The hardness is determined by scratching the mineral in question by the minerals in Mohs scale.
Color

Gem mineral show off their charm and appeal through their color. Color is an appearance indicator of a mineral in reflected light and transmitted light of petrographic polarizing microscopes for translucent specimens. Hand specimens possessing deep colors will appear faintly colored in thin sections, when those that appear opaque in hand samples will exhibit variety of color when cut into sections. At times colors may be given to a section simply by the presence of great numbers of minute inclusions.

The light transmitted by the different minerals varies both in form and kind. Some minerals may appear opaque while others maybe perfectly transparent. Colorless minerals usually allow all light constituents to pass through. Some minerals may readily absorb some portions of the spectrum and as a result, the light reaching the eye exhibits variety of colors. These color are known to be complementary to the light absorbed. This color can be use for identification of the rock-forming mineral in question.

Some cases may include thin sections containing several fragments of a different mineral. Variations in the color of different fragments are intimately related to crystal symmetry. If the mineral in question happens to crystallize in cubic system, all the fragments present in the section will be either of the same color or colorless. In transmitted light of polarized microscope for geologists, all sections of one mineral with crystals formed in cubic system will commonly exhibit colorless appearance or of the same colors. Those that crystallized in other systems usually shows different colors for different sections but may also appear colorless or with the same colors.

When a colored mineral is viewed reflected light of polarizing microscope and no color change as the stage is rotated, the colored mineral is said to be isotropic. On the other hand, Anisotropic means, the minerals exhibits a change in color in varying degrees as the stage of a polarized light microscope is rotated.

In another point of consideration, the examination of the mineral in thin section with the use of the lower nicol or polarizer, the light reaching the section has previously passed through the polarizer. Then mineral under examination when placed on a stage and rotated for about 90?, there will be a change in color exhibited. The change in color shown by the mineral is called pleochroism. It is considered as a valuable means of distinguishing many minerals with the polarizing microscope. It must be understood however that the strength of the pleochroism depends on the direction in which the section is cut.

Certain minerals, otherwise colorless in thin section, show curious brown areas, each of which contains at its center an included fragment of some other mineral. These brown patches change in color when the section is rotated above the polariser of petrographic polarizing microscope. These brown patches are infact, pleochroic areas in an otherwise colorless section. It is quite certain that the including mineral has acquired this property in consequence of the presence of this minute inclusion. It has been known and proven that the development of color around the inclusion is the result of the fact that such an inclusion is radioactive, and that its ejected particles have produced an ionization effect on the mineral in contact.

The practical way of testing pleochroism is by revolving the stage of a polarized light microscope, carrying the section, when a change in color of the mineral is found, then it is pleochroic. This pleochroism may appear as an actual change in color or simply as a change in shade of the same color. At times it may be so weak and hardly to be noticed. It will be best to adjust the aperture diaphragm of the petrographic polarizing microscope or by rotating the analyzer.

Luster

Luster indicates the appearance of the mineral surfaces in reflected light when viewed under polarizing light microscopes. Metallic and non-metallic luster are two principal types of luster.

Substances with metallic appearance show metallic luster. They are also considered as opaque.

Non-metallic luster on the other hand is observed on gem minerals like

Adamantine – Observed to be that luster typical to diamond and of other substances with high refraction indices.
Vitreous - Observed to be the luster of glass.
Resinous - The luster of resin.
Greasy - The appearance similar to a surface with oil.
Pearly – The luster shown by the mother-of-pearl.
Silky - The luster shown by the fibrous minerals.
Dull - The luster neither bright nor shiny.

A substance that comes in between metallic and non-metallic luster may show sub-metallic luster.

Cleavage

Cleavage is defined as the ever-present ability of a mineral to split or break along definite plane. Cleaving a mineral can be done by pressing the edge of the knife blade upon it or by simply striking it properly directed. The description of cleavage is by indicating the crystal face parallel to which it takes, the character of the resultant surface, and the ease of obtaining the cleavage. When viewed under a polarizing petrographic microscope, the presence of cleavage is recognizable by the frequent appearance of cleavage cracks within the crystal or by the surfaces that appear to be irregular character like the number of small parallel, steep-like cleavage planes. Cleavage planes are planes along which the breaking takes place. Cleavage faces are the resulting surfaces after the separation occurs. Petrographic polarizing microscopes can detect the development and appearance of cleavage planes in finished thin section as lines of varying width after grinding or cutting the mineral. Cleavage appears more or less distinct and regular lines or cracks. These cleavage cracks may be parallel or intersect, depending on the position of the section relative to the cleavage planes of the crystal. They can be observed by slightly adjusting the aperture diaphragm of petrographic polarizing microscope.

Streak

The streak of a mineral is the color of its powder after rubbing it over a piece of white unglazed porcelain called streak plate.

Tenacity

This property refers to the extent of mineral resistance to breakage or to being crushed. High or low degree of tenacity shows the toughness or brittleness of the substance. When the gemstone is subject to pressure, tenacity will defend on how the substance is cleaved, parted, or fractured.