Semi-Precious Stones - A Historical Article on Agate, Amber, Amethyst and Many Other Varieties of Gemstones

By Edwin W. Streeter

This book contains classic material dating back to the 1900s and before. The content has been carefully selected for its interest and relevance to a modern audience. Carefully selecting the best articles from our collection we have compiled a series of historical and informative publications on the subjects of gemology and crystallography. The titles in this range include "Gemstone Manufacturing" "Geometrical Characters of Crystals" "The Thirty-Two Classes of Crystal Symmetry" and many more. Each publication has been professionally curated and includes all details on the original source material. This particular instalment, "Semi-Precious Stones" contains information on amber, amethyst, agate and much more. Intended to illustrate the main varieties of semi-precious stones it is a guide for anyone wishing to obtain a general knowledge of the subject and to understand the field in its historical context. We are republishing these classic works in affordable, high quality, modern editions, using the original text and artwork.

• Language: English
• Publisher: Read Books Ltd.
• Release date: Jul 7, 2014
• ISBN: 9781473394360

Book preview

SEMI-PRECIOUS STONES.

Of less commercial value than those described in the foregoing pages, but nevertheless many of them very beautiful.

IT is deemed advisable to arrange this large series of Stones in alphabetical order, without expressing any opinion as to their relative values. Many of these Stones were formerly much worn, but at present there is only a slight demand for them. In my opinion however, some of them, especially the Amethyst, will again become fashionable.

The coloured plates represent several of these stones, shewing their crystalline form, which it is hoped may serve as a guide to those who are interested in the study of gems.

THE AGATE.

BY the term Agate, the mineralogist understands a composite substance, an association of certain silfceous or quartz-like minerals, which in texture, colour, and transparency are diverse one from another. These Agate-forming minerals are chiefly Chalcedony, Carnelian, Jasper and Quartz. Two or more of these, forming a variegated stone, and usually presenting a diversity of spots and stripes, may be denominated an Agate. The name is derived from the river Achates, in Sicily, now known as the Drillo, in the Val de Noto, wherein, according to Theophrastus, the ancient Agates were found, in his time.

The Agate is occasionally found in veins, as in certain localities in Saxony and Bohemia, but, as a rule, it occurs in the form of nodules embedded in an amygdaloidal rock, more or less akin to basalt.

On the decomposition of the amygdaloidal agate-bearing rock, the enclosed Agates, by reason of their resistance to the disintegrating effects of weather, remain behind as nodules; hence Agates are frequently found loose in the beds of rivers. The Scotch pebbles are Agates which have been liberated by decomposition of their matrix of porphyrite, and are found scattered over the surface of the ground.

Various theories have been propounded from time to time, for the purpose of explaining the origin of the Agate nodules in the cavities of the rocks wherein they occur. The cavities themselves have unquestionably resulted from the imprisonment of gas bubbles, whilst the rock was in a molten condition. The agate-bearing rock is, in most cases, an ancient lava. The nodules of Agate are considered to result from the crystallization, or non-crystalline deposition, of silica, from a solution with which the cavity of the nodule or geode became filled. The silica—now in one condition, such as Jasper, now in another, such as Chalcedony, and then again in the crystallized form of Quartz—was deposited over the irregular inner surface, giving rise to those concentric markings which are seen on the sections of most Agates. This deposition of silica would continue until the geode became filled so as to form a solid Agate, or the inlets of infiltration became stopped up, or the supply of siliceous solution failed. In other cases the silica would be deposited on the walls of the cavity in concentric layers, while, after a time, owing to some change in the natural conditions, the silica might be deposited in layers on the floor of the cavity, in obedience to gravitation, and the various coloured bands would then run parallel to each other in horizontal layers.

According to certain fancied similitudes, which the Agate stone displays to things in common use, it receives distinguishing names. Thus Riband Agate exhibits strata or layers of different colours which play one into the other. If the stripes of varied hues are arranged round the centre, it receives the name of Circular Agate; and if in this centre there are other coloured points, it is called Eye Agate. When the variously coloured bands are disposed in an angular pattern, suggestive of the plan of a polygonal fortress, it is called Fortification Agate. Moss Agates enclose green and brown mineral matter suggestive of vegetable growth, whilst Mocha Stones contain dendritic or branching markings of brown colour, due to oxide of manganese and perhaps