538 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



arises a number of different colour-varieties of amber, which are classified into two groups 

 under the descriptions " variegated osseous clear " and " variegated osseous bastard." 



Frothy amber is opaque, very soft, and incapable of receiving a polish ; it often encloses 

 crystals of iron-pyrites in large numbers. 



Leaving out of consideration those colour-varieties of amber which are rare or unusual, 

 the pearl-coloured, and next to this the " kumst "-coloured, is the rarest of the varieties of 

 €very-day occurrence. These two kinds of amber are in more general favour in Europe than 

 is the clear amber, and for this reason command the higher prices, but we shall see later on 

 that the fashion and taste prevailing in different countries is not uniform. 



The different degrees of turbidity in amber, with which are connected the differences 

 between the numerous varieties of this substance, were at one time thought to be due to the 

 presence of varying amounts of water. It is now known, however, that the turbidity is due 

 to the enclosure of vast numbers of bubbles of various sizes, but always too small to be seen 

 with the naked eye, or with a simple lens, their identification requiring the examination of 

 thin sections under a high power of the microscope. These cavities are distributed 

 throughout the ground-mass of the amber, which always consists of a pure clear resin, 

 almost water-clear to reddish-yellow in colour. The differences in the appearance of the 

 various kinds of amber depend upon the number and the size of these cavities, the smallest 

 of which have a diameter of 0*0008 and the largest 0'02 millimetre. The cavities are the 

 smallest and the most numerous in osseous amber, having a diameter of from 0"0008 to 

 '0"004 millimetre and a distribution of 900,000 per square millimeter. " Flohmig" amber 

 contains the smallest number of bubbles, 600 per square millimetre, but these, with a 

 diameter of 0'02 millimetre, have the maximum size. Between these two extremes lie all 

 the other varieties, the study of which has shown that the more numerous and the smaller 

 are the bubbles the amber contains the more turbid it becomes. When the cavities are 

 less numierous and larger in size the amber in which they are embedded is clearer, and when 

 it contains no bubbles is perfectly transparent. 



The small bubbles of air hinder the passage of light through the amber and thus 

 produce the appeai'ance of turbidity. A large portion of the light which enters a mass of 

 amber is reflected from the surface of the air bubbles and thus fails to reach the eye. 

 Could these air-spaces be filled with a transparent substance of the same, or nearly the same, 

 refractive index as amber, which varies between 1-530 and 1-547, the turbidity would 

 •disappear and the substance would become perfectly transparent. 



This, in fact, can be accomplished without any great difficulty by an operation which is 

 known as clarifying the amber, one which is frequently performed in the trade for the 

 purpose of making cloudy amber transparent and thereby increasing its value. The rough 

 material is completely immersed in rape-seed oil in an iron vessel, and then very slowly 

 heated to about the temperature at which the oil boils and begins to decompose. It is then 

 allowed to cool and this must take place just as slowly and gradually as the preliminary 

 heating, otherwise the clarified amber will become cracked and possibly fractured. The 

 smaller the fragments of amber operated upon the quicker is the process completed ; the 

 heating of large pieces must be continued for a considerable period, and not infrequently 

 needs to be several times repeated. The time required for the operation depends also upon 

 the character of the material, for different pieces of amber of the same size will not require 

 the same length of time to complete the operation. The clarifying process begins on the 

 surface and spreads gradually inwards. 



The rationale of the method is the penetration of the fine cracks in the amber and the 

 fining up of the cavities with rape-seed oil, the index of refraction of which is 1-475, only 



