A Guide to Birthstones - A Collection of Historical Articles on the Gemstones Linked to Astrology

Various

Description

A comprehensive guide to the main varieties of birthstones, including their properties and structure.

This volume features extracts from eight historical articles on the subjects of gemology and crystallography. This illustrative guide to birthstones is an ideal read for those who are interested in learning more about their spiritual connection to the natural world.

Featured extracts taken from the following articles:

- ‘Gems and Gem Materials’ Edward Henry Kraus

- ‘The Natural History of Gems – Or, Decorative Stones’ C. W. King

- ‘Diamonds and Precious Stones - Their History, Value and Distinguishing Characteristics’ Harry Emanuel

- ‘A Manual of Precious Stones and Antique Gems’ Hodder M Westropp

- ‘Precious Stones and Gems - Their History, Sources and Characteristics’ Edwin W. Streeter

- ‘Gem Testing’ B W Anderso

- ‘A Hand-Book of Precious Stones’ M. D. Rothschild

- ‘Gem Stones and Their Distinctive Characters’ G. F. Herbert Smith

Sujets

• Horoscopes
• Spirituality
• Astrology

Informations

• Publié par: Read Books Ltd.
• Date de parution: 16 septembre 2016
• Nombre de lectures: 0
• EAN13: 9781473356412
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

A Guide to Birthstones
A Collection of Historical Articles on the Gemstones Linked to Astrology
By
Various Authors
Copyright 2011 Read Books Ltd. This book is copyright and may not be reproduced or copied in any way without the express permission of the publisher in writing
British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library
Contents
Aquarius - January 21 to February 19 - Garnet
Gems and Gem Materials . Edward Henry Kraus
Pisces - February 20 to March 20 - Amethyst
The Natural History of Gems - Or, Decorative Stones . C W King
Aries - March 21 to April 19 - Bloodstone or Heliotrope
Diamonds and Precious Stones - Their History, Value and Distinguishing Characteristics . Harry Emanuel
Taurus - April 20 to May 20 - Sapphire
A Manual of Precious Stones and Antique Gems . Hodder M Westropp
Gemini - May 21to June 21 - Agate
Precious Stones and Gems - Their History, Sources and Characteristics . Edwin W Streeter
Cancer - June 22 to July 22 - Emerald
Gem Testing . B W Anderson
Leo - July 23 to August 23 - Onyx
A Hand-Book of Precious Stones . M D Rothschild
Virgo - August 24 to September 22 - Carnelian
Diamonds and Precious Stones - Their History, Value and Distinguishing Characteristics . Harry Emanuel
Libra - September 23 to October 22 - Peridot
Gem Stones and Their Distinctive Characters . G F Herbert Smith
Scorpio - October 23 to November 22 - Beryl
Gems and Gem Materials . Edward Henry Kraus
Sagittarius - November 23 to December 21 - Topaz
Gem Stones and Their Distinctive Characters . G F Herbert Smith
Capricorn - December 22 to January 20 - Ruby
A Hand-Book of Precious Stones . M D Rothschild
GARNET
Though the term garnet is often understood as referring to a single gem, in reality the name includes a number of closely related minerals. Several chemically similar elements freely replace one another in the garnet group. As a result, the properties of the group are variable, and consequently garnet gems may be quite different in appearance.

F IG . 301.-Garnet (rhombic dodecahedron). Salida, Colorado.
Garnets are in fact very attractive stones, but they are so abundant that they are not highly valued. Red garnet is frequently sold under deceptive names, such as Cape ruby and Arizona ruby , and the green garnet from Russia is known as the Uralian emerald . This undesirable practice has a tendency to increase the price of the gems masquerading under such titles (p. 139).
All varieties of garnet crystallize in the cubic system with very similar forms. The rhombic dodecahedron ( Fig. 301 ) and the tetragonal trisoctahedron ( Figs. 302 and 303 ), alone or in combination ( Figs. 304 to 306 ), are the only commonly occurring forms. Although the mineral is generally well crystallized, it may occur in compact granular aggregates, in rounded and disseminated glassy grains, or as water-worn pebbles. Garnet has an imperfect rhombic dodecahedral cleavage, and a conchoidal to uneven fracture. The hardness varies from 6 1/2 to 7 1/2, and the specific gravity from 3.4 to 4.3, depending upon the composition.

F IG . 302.

F IG . 303.-Garnet (tetragonal trisoctahedrons) in mica schist. Sunday River, Maine.

F IG . 304.-Wooden models showing combination of rhombic dodecahedron and tetragonal trisoctahedron.

F IG . 305.-Rhombic dodecahedral crystals of garnet.
Almost any color except blue may be observed in garnet. Red, brown, yellow, green, and black are the more common colors. It may also be colorless. Although the colors are largely due to the elements entering into the composition of the various members of the group, they may be attributed in part to impurities present in minute quantities. The lighter colored garnets are generally transparent to translucent; the dark-colored varieties, translucent to opaque. The luster is vitreous to resinous.
The index of refraction of gem garnets is as variable as the other properties, lying between 1.70 and 1.94. The dispersion ranges from 0.024 to 0.028 for most varieties. However, one variety, the demantoid or grass-green andradite, has a dispersion of 0.057, which is comparable to that of the diamond. Hence, when the color is not too dark, these garnets have considerable brilliancy and fire.

F IG . 306.-Crystal of garnet showing a combination of rhombic dodecahedron (large) and tetragonal trisoctahedron.
As garnets are isotropic, they do not show dichroism. This fact aids materially in differentiating red garnets from rubies, which possess similar indices of refraction and specific gravities.
The composition of the members of the garnet group is easily expressed by the generalized formula M 3 M 2 (SiO 4 ) 3 , in which M represents any of the bivalent metals magnesium, calcium, manganese, or iron and M a trivalent metal, aluminum, ferric iron, or chromium, for example, grossularite Ca 3 Al 2 (SiO 4 ) 3 or 3CaO.Al 2 O 2 ,3SiO 2 (see table below). A small amount of titanium may also be present. Thus, a garnet may contain all these bivalent and trivalent metals in almost any proportion, so long as the ratio between the bivalent and trivalent elements is 3 to 2. However, based upon composition, six varieties of garnet have been established. The composition of these varieties grades over into one another, and, consequently, also the various physical and optical properties. The composition of a garnet is hence commonly much more complex than that theoretically assigned to the variety to which it belongs. In the table below, the composition, specific gravity, and index of refraction n of each of the several varieties are given:

The varieties of garnet will now be discussed, especially with regard to their use as gems.
Grossularite. -The lime-aluminum garnet is lighter colored than the other varieties; in fact, it is sometimes colorless. However, pale emerald green, rose red, or various shades of yellow and cinnamon brown are most commonly observed. The yellow- and cinnamon-colored stones are the ones frequently used as gems, under the names cinnamon-stone, hessonite , or essonite . The terms hyacinth and jacinth , the latter more properly applied to yellow or brown zircon, are also used for these stones. The green- and rose-colored varieties are rarely transparent enough for cutting. Hessonite has a peculiar granular structure, visible even in the cut stones. The green massive grossularite, often mottled green and white, from Buffelsfontein, Transvaal, is called South African or Transvaal jade . The mineral chromite is often associated with it.
Grossularite is characteristically found in metamorphosed, impure limestones, associated with vesuvianite, wollastonite, diopside, scapolite, and other lime silicates. Most of the hessonite is obtained from Ceylon, where it occurs with zircon in gravels.
Pyrope. -This is the most popular garnet, probably because it frequently has a beautiful ruby-red color, which is due to impurities. Pure pyrope would be colorless. Pyrope is the variety occurring with the diamond in South Africa, where it is sold as Cape ruby (p. 185). It is not often found in good crystals but usually occurs in grains or small irregular masses or as stream pebbles. The color varies from deep red to black, often with an undesirable tinge of yellow. Rhodolite is rose red or purple. Its composition and properties are intermediate between pyrope and almandite. The index of refraction is 1.760. It occurs in Macon County, North Carolina.
Pyrope occurs chiefly in basic igneous rocks, such as peridotite, and in the serpentines derived from them. Czecho-Slovakia, Saxony, South Africa, Arizona, and Colorado are important localities. Rose-cut pyrope is used in Bohemian jewelry. This type of garnet is commonly observed in antique brooches, necklaces, and so forth.
Spessartite. -This variety is not commonly used in jewelry, since ordinarily its color is an unpopular brown. The color may also be brownish red, orange red, or dark hyacinth red with a tinge of violet. It occurs in granite and quartzite and with topaz in rhyolite. Bavaria, Tyrol, Ceylon, Virginia, and Nevada are some of the localities for this variety.
Almandite. -The almandite garnet was known to the ancients. It includes the once popular carbuncle . The color of almandite varies from deep red and violet red through brownish red to almost black. The transparent red varieties are used as gems. However, most almandite is too dark and opaque for that purpose and accordingly on account of its superior hardness finds use as an abrasive. In artificial light almandite occasionally has an orange hue. The absorption spectrum is strong, and contains a pronounced band in the yellow, which accounts for the violet hue of some stones (p. 79). The cabochon cut is most used for the carbuncle, the darker stones often being cut in the hollow cabochon style, whereby a more attractive color is obtained. On some carbuncles a four-rayed cross of light is seen, constituting an interesting type of asterism.
Almandite is found in a variety of rocks, including gneiss, schist, granite, pegmatite, and other igneous rocks.