This assignment consists primarily of two comprehensive charts that summarize the important properties and characteristics of the materials that may be encountered in the jewelry industry. Chart I is meant to represent those stones likely to be found in a Jeweler's stock, and Chart II lists those stones that are rarely found in jewelry. Many of those listed on Chart II are very rare and are seldom seen by the jeweler; however, they are included to make the chart as complete as possible. Although there are a few additional stones discussed in some tests under the heading of gem materials, they are almost never used for jewelry purposes.
The charts are excellent property tables for testing and are intended for use on well or desk. It is recommended that they be covered with glass or plastic, or otherwise protected against damage and dirt. Of course, they will not eliminate all reference to assignments and textbooks, but when you become familiar with them, the reference will be greatly simplified.
Study carefully the following instructions pertaining to use of the charts and the interpretation of various symbols used thereon.
Column I: Species
This column lists: species names of the various gem
materials. Group names are included where applicable. It will be
necessary to use a different reference for varietal names for these
species.
Column II : Color Range
Almost any color system for describing gemstones is inadequate because of the wide variations in hue, tone and intensity that are encountered in most gem species. This column has been color keyed for an easier reference. The letters C,- P, R, 0, Y, G, B, V. Br, W, Cr and BI mean colorless, purple, red, orange, yellow, green, blue, violet, brown, white, gray, and black, respectively. As you can see, this does not represent the electromagnetic spectrum, but rather the basic colors in which these stones occur. The color pink is not included, however; pink should be treated as simply a lighter tone of red. The colors indicated should not be used as that stone's key color in many cases, for the colors indicated for a particular stone may indicate a mixture of colors. For example, we would not expect to find almandite garnet in a pure hue of purple, red or brown, but, instead, it is usually a mixture of two or three of these colors.
Horizontal and vertical lines will be noted in each of the colors indicated for a particular stone. If the line is horizontal in a certain color this indicates that the stone is generally found in that color. When a atone occurs only rarely in a color, the line in that color is crossed by a vertical line. Also, it should be noted that, when the situation allows, the horizontal line will extend only partially into a certain color or colors, indicating a mixture of colors. (See synthetic emerald: You will find the line extends all the way through green and blue and partially into the bordering color of yellow. This indicates that the stone will be found primarily in a yellowish or bluish-green color, green being predominant.)
Column III: Transparency
Although any gem material can occur in such a badly flawed or imperfect state that it appears opaque to the eye, the opaque classification refers only to materials that are normally opaque, e.g., turquoise, hematite and malachite. The symbols Tp, Ti, and O are interpreted as transparent, translucent and opaque, respectively.
If a stone is important only when it is transparent, only the symbol Tp is entered in this column. On the other hand, if a species has some varieties that are important only when transparent or nearly flawless, but it also has other varieties that display phenomenal characteristics such as asterism or chatoyancy, the column then shows Tp - Ti, meaning that the stone can occur in a transparent to translucent form. Likewise, Ti - O means that gem qualities of a stone may range from translucent to opaque; this is typical of various aggregates such as jade, chalcedony and shell. If a stone has only the symbol O in this column, it occurs in opaque form only; this is typical of hematite, jet and malachite.
Column IV: Refractive Index and Birefringence
This column shows the typical refractive indices encountered in the various gemstones. Since most species show slight differences in R.I., depending on variations in composition and structure, these figures must be accepted with this variability in mind. (The possible ranges resulting from these variations are listed in the column entitled "R.I. Range.")
When only one index is listed, it will be seen with a stone which is also singly refractive; therefore, only one index is possible. In a doubly refractive stone, the principal R.I.'s are shown, followed by the stone's birefringence. The determination of birefringence is one of the most important tests made on the refractometer. An R.I. determination which is only an approximate figure has much less value than readings which measure birefringence. Unlike most other properties, birefringence is a rather constant figure for most doubly refractive stones. It is possible, for example, to obtain an index of approximately 1.62 for a stone that actually could be tourmaline, topaz, glass, or one of the unusual gemstones. However, determining the degree of birefringence, or its absence, would separate the existing possibilities immediately. Usually, only one reading predominates in an aggregate, even though it is doubly refractive.
The shaded area that extends from the top of both charts down through and including spessartite on Chart I and gahnite on Chart II indicates that these stones are over the limits of the normal refractometer (1.81 +). The shaded lines extending up Chart I from glass, plastic and assembled stones indicate the possible range in R.I. of these materials.
Column V: R.I. Range
The R.I 's shown in the column entitled "Refractive Index and Birefringence" are usually the typical figures for the species in question. However, there are at least two other possible situations shown. Topaz and beryl, for example, have different varieties with distinctly different Indices. In the interest of simplicity, the figures shown represent midpoints between the two range; if the usual R.I.'s for all or most varieties are near one end of the total range, the usual reading or readings is/ are shown, followed by two different figures for possible variations from that. If the usual figure is in the middle of the range, the variation is shown in one plus-and-minus figure.
In doubly refractive stones, do not expect one index to vary toward the high limit and the other toward the low. For example, beryl is listed at 1.577 - 1.583, with an extreme range of ±.017. Thus., beryl could be found with indices of 1.560 - 1.566 or 1.594 - 1.600, but not 1.560 - 1.600. (See the assignment on property variations for further details.) Possible variations of the two with respect to one another are shown by the limits of birefringence in the column "Refractive Index and Birefringence."
Column VI: Crystal Systems and Optic Character
The crystal system to which each gem species belongs is listed in this column, in addition to an abbreviation for the basic classification of single or double refraction. Since the polariscope reaction for aggregates varies from that of a single crystal, stones that usually occur as aggregates are designated as AGG. If a stone is shown as DR (double refraction) in addition to AGG. this means the stone can occur as an aggregate as well as a single crystal. (In this latter case, the stone would show a typical doubly refractive reaction.)
Optic character is also included in this column. This is indicated by the letter U for uniaxial stones (found in the hexagonal and tertragonal systems) and B for biaxial stones (found in the triclinic, Orthorhombic and monoclinic systems). The plus (+) and minus (-) figures (optic sign) indicate whether the stone is optically positive or negative. Optic character plays a very important role in identification.
Column VII: Pleochroism
Stones that show only dichroism have a plus sign under the column designated as 2. Those that display only trichroism have a plus sign under column 3. Sometimes, however, the third color of a trichroic stone is so similar to one of the other two colors as to be confused with it. There is also the possibility that a species may show either dichroism or trichroism. In this case there will be a plus sign under both columns 2 and 3.
The last columns merely indicate strong, moderate or weak pleochroism. Stones that can display strong pleochroism may also have a plus sign in the moderate and weak column, since lighter colors or tones of these species obviously display weaker pleochroism.
Column VIII : Specific Gravity
The possibility of variation in the specific gravity listed for the majority of stones should be kept in mind. The plus-and-minus factors will probably have to be considered in many cases. The variation in a stone's S.G. can be judged by the plus-and-minus figures which follow the S.G. in this column. For example, quartz, with a 2.66 S. G., is quite constant, with only a plus-or-minus .01 variation. On the other hand, the S.G. of lapis with a + .25 should not be used as an important test, if at all.
In many instances, a fluctuation in S.G. is due to the different varities of a particular species. For example, as the stone spinel approaches gahnospinel ( a high zinc content spinel), the S.G. will rise appreciably. Another example is garnet: the S.G.'s for most of the garnets will vary considerably, because many are mixtures of more than one garnet species. Other factors that may cause S.G variation are the presence of inclusions and variation in chemical composition. Also, very porous or badly flawed stones may exceed the range given. If S.G. is obtained by the hydrostatic method and the stone is under .30 carat, the result may be off appreciably beyond the range given in this column.
Column IX: Spectra
This column must be used in conjunction with the SUPPLEMENT or other references where physical spectra can be noted. Most of the spectra are evaluated by one, two or three starts, a system adapted from that used by B.W. Anderson. In this system, the following meanings are indicated: 1) If a stone shows a three-star spectrum, this indicates that the spectrum is always present and diagnostic in this color. 2) A two-star spectrum is diagnostic when present, but it ii; not always present or clearly defined. 3) A one-star spectrum is sometimes useable as a confirmatory test, if present. 4) If no star is used in this column, but simply an S is listed, the spectrum represent one that has been recorded in either of gem laboratories but is either rarely seen or too few stones have been examined to be able to indicate value in testing.
Some two-and three-star-spectra represent very few stones examined in rare species or colors; but the presence of certain essential coloring agents that cause characteristic absorption makes for a safe assumption of diagnostic value. If no indication of spectra is given, the stone either shows no spectrum or it is useless in identification because of enormous fluctuation.
Column X : Dispersion
Comparative dispersion of various gemstones is shown in this column. The numerical figures indicate the difference in refractive indices in the red and blue-violet wavelengths of the spectrum. The measurement of dispersion is generally not used as a gemological test; however, in many instances it ca n be used as a separation factor in identification.
The best way to quantify dispersion would probably be on a high, moderate, and low basis. For example, if we had a blue, highly dispersive, doubly refractive stone, over the limits of the refractometer, this would be very indicative of synthetic rutile. Where the stone zircon, which is most often confused with synthetic rutile, it would have a moderate dispersion, at best. On this basis, a good indication is present for separation based on dispersion.
Column XI: Phenomena
The various types of phenomena are indicated by the following symbols: F for fluorescence; A for asterism; CC for change of color; C for chatoyancy; I for iridescence; O for orient; A V for aventurescence; AD for adularescence; P for play of color; L for labradorescence.
Column XII : Moh's Hardness
The hardness figures listed in this column are based on Moh's scale. See the Colored Stones hardness for further reference.
Column XIII: Acid
This column indicates the reaction of a specimen to one of the common acids, such as hydrochloric, nitric or sulphuric acid. If a stone is not attacked by acid., it is listed as N for negative. If acids attack it, but not violently, it is merely listed as A for-attacked. An E indicates that a stone is attacked readily to the extent that it will effervesce. If a stone is attacked by one acid only, the formula for that add is given: HF for hydrofluoric, H2SO4 for sulphuric, HCL for hydrochloric, and HNO3 for nitric.
Column XIV: Cleavage
This column lists cleavage only for those stones which show it readily. Since these gemstones are likely to cleave, presence of cleavage in a gem may be useful in Its identification and therefore is indicated on the chart.
Column XV : Additional Characteristics
It is important to note that these miscellaneous characteristics are not necessarily encountered in every specimen. In other words, such features as typical inclusions and, luster may be seen easily in a sufficient number of specimens of a given species to make them important as identifying characteristics. However, inclusions are obviously variable, and many stones may be flawless.
Also included in this column are names of those stones usually confused with one another, and the key separating factors used for identification are given.
