It has been stated that when light traveling in air strikes a denser medium, such as a gemstone, a portion of the beam is reflected from the surface and a portion is refracted into the stone. The quantity reflected and the quantity refracted depends on the refractive index of the stone and the flatness of its surface. Of course, the angle at which the light beam strikes the surface (i.e. the angle of incidence) has a bearing on the quantity reflected versus the quantity refracted; but in comparing the luster of two stones, the angle of light incidence and the viewing angle must be kept the same on the stones being compared. The denser the gem optically (in other words, the higher its R.I.), the greater the surface reflection. This can be visualized as being caused by light bouncing off more compactly spaced pattern of atoms. If the surface is not polished to near flatness, optically, the light is diffused to a greater degree. The greater the angle to the perpendicular of the approaching light (i.e., the more nearly parallel to the surface), the greater the proportion reflected. This can be likened to a flat stone on water. If the stone is dropped straight into the water, it sinks, but it can be skipped across the water if it strikes the surface at a flat angle.
Luster, which is affected by the above mentioned factors, may be defined as the appearance of a surface when it reflects light directly to the eye. The classification of luster depends on the following factors:
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Quality of Reflected Light
This factor is determined by the character of the reflecting surface, and may be subdivided into two general classes: (I) PLANE REFLECTION (reflection from a flat, highly polished surface) and (II)DIFFUSED REFLECTION (reflection from a dull surface such as sandblasted metal).
There are vast differences in so called plane-polished surfaces that are smooth to the unaided eye or even to a 10x loupe. Stones polished too rapidly usually have minute parallel grooves that are visible under 10x. Even carefully polished stones of many species, because of brittleness or differences in hardness with direction, do not have smooth surfaces. Surface irregularities affect luster materially, by causing a beam of light to be diffused rather than reflected at one angle.
Sketch A in Figure 9 shows reflection of light at the same angle from a plane polished surface; sketch B shows slight diffused as a result of poor polish; sketch C shows highly diffused reflection from a rough, irregular surface.
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Quantity of Reflected Light
Of all the light falling on surface, the proportion reflected depends on the refractive index of the substance and the angle of incidence at which the light strikes the surface. The greater the angle of incidence and refractive index for any given stone, the greater the quantity of light reflected. Metals, because of their very high refractive indices, reflect almost all of the light that strikes them.
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Body Appearance
Some minerals exhibit an internal formation, or body structure, that affects their general appearance in reflected light. Since this structure seldom has any effect on the appearance of the polished surface, in gemology it is mentioned separately as body appearance. An example is the fibrous structure of tiger's-eye, which gives a rough specimen of this material a silky appearance. When polished, tiger's-eye loses the silky appearance on its surface, but the effect is still visible in the body of the stone. This effect of body texture on surface luster is known as SHEEN.
The luster of any material, as pointed out above, depends on both
the quantity and quality of light reflected from it. The majority of
gems can be polished to present very nearly perfectly flat surfaces,
and they will therefore show plane reflection. Some, however, because
of their structure, can not be polished to produce a truly flat surface.
Jade, other than the finer qualities, is an example of the latter. The
following are the principal kinds of luster:
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Metallic
This luster, which is shown by metals and a very few gem materials, notably pyrite and, marcasite, is the most brilliant of all. It results from the plane reflection of a great proportion of all the light that falls on a surface.
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Adamantine (diamond like)
Adamantine refers to the surface appearance of those gemstones that exhibit plane reflection of a fairly large proportion of all the light that falls on them. Diamond is the outstanding example. Of the light that falls from all angles on a plane-polished surface of a diamond, approximately 30% is reflected. Other gems that approach an adamantine luster are zircon and demantoid garnet.
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Vitreous (glasslike)
This luster is characteristic of those gems that exhibit plane reflection but that reflect only a small proportion (less than 30%) of the light that falls on them. The majority of gems show a vitreous luster. Examples are beryl, quartz, topaz and others with refractive indices between 1.50 and 1.70.
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Greasy and Resinous
These luster are characteristic of those polished surfaces that are not perfectly flat but that approach flatness. Examples of a greasy luster are some rough garnets and peridots. However, these two gem materials, when carefully polished, may be made to exhibit very nearly perfectly plane surfaces; therefore, they can show a vitreous luster. The usual commercial polishing methods used for jadeite and nephrite do not produce a perfectly flat surface; consequently, polished specimens have a slightly greasy luster.
The term resinous is sometimes applied to the more or less greasy luster of yellow resins or to any other yellow material resembling resin. Although a resinous luster can be seen on a fractured surface of amber, it can be polished to show a surface that is quite plane and thus exhibit a very nearly vitreous luster.
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Waxy
A waxy luster resembles greasy and resinous luster, except that the surfaces on which it is seen are more irregular Examples are poorly polished turquoise and most rough chalcedony.
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Dull
A dull luster can be described as the appearance produced by a very irregular, fine-grained surface; i.e. a surface composed of a multitude of minute projections. A dull luster is typical of rough turquoise. It is never seen on well-polished surfaces of gems.
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Silky
This luster is produced by a fibrous body appearance. It is best seen on rough specimens of tiger's eye. Polished tiger's eye shows a vitreous luster on its surface, overlaying a fibrous body, and the resulting optical effect is known as a SILKY SHEEN.
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Pearly
Luster is described as pearl, when the appearance is similar to that of a fine pearl. The only gem mineral with a luster that can be described as pearly is rough moonstone, the characteristic sheen of which is so named for want of a more appropriate term.
The most important luster of polished gemstones are adamantine and vitreous. It should be noted, however, that there is not a sharp dividing line between different luster. No two gemstones have exactly the same luster. If all gemstones could always be polished to equally flat surfaces, luster would depend only on refractive index. Since this is not true, some gemstones have higher or lower luster than their refractive indices would lead one to expect. Topaz and almandite garnet both take an excellent polish and have luster somewhat higher than their positions on the R.I. scale would suggest. Synthetic rutile and usually zircon also have luster below what their high R.I. would suggest.
To observe the luster of a gem, hold it toward the light from a window or from a diffused artificial light source such as a frosted light bulb. The intensity of the light, of course, will affect the intensity of the luster. If luster is being used for identification, the light source must not be of high intensity. The surface should be observed both at an angle that allows the light to be reflected directly to the eye and away from the eye.
The brilliancy of a transparent gemstone depends on the intensity of its luster plus other factors to be mentioned later. Since brilliancy is a form of beauty that is admired by the majority of persons, high intensities of luster are extremely desirable. The brilliancy of diamond is a principal factor in making it one of the most desirable of all gems. In other varieties of transparent stones, the highest luster possible is almost always the most desirable. Gemstones fashioned in the Orient are usually deficient in polish hence, luster is affected. The subdued luster of some turquoise and jade appeal more to some people than a more brilliant luster.
The luster of diamond is so distinctive that it is by this property
that it can be immediately identified by those who have examined many
stones. However, a poorly cut and polished diamond, compared with a
well cut and highly polished zircon, observed at a short distance in
a brilliant light and judged by luster alone, might deceive even an
expert. Luster as a means of identification is valuable in separating
a well-polished stone of adamantine luster or one with a greasy luster
(such as jade) from other gems. If stones are poorly polished, their
characteristic luster are often more noticeable, but the variations
in polish make luster a doubtful means of identification, except to
the more experienced. A knowledge of the luster of rough and broken
minerals assists the gemologist in identifying uncut stones. Also fractures
on cut stones reveal broken surfaces that can be examined for luster
under a loupe. This is especially useful in separating turquoise and
chalcedony from glass imitations. The luster difference between the
two parts of a garnet and glass doublet is also useful: a very definite
difference between the garnet top and the glass base can be noted, usually
on the crown facets.
