422 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



this is of a rich oil-green shade, and, when cut, may closely resemble chrysoprase in colour 

 and lustre. The two stones may be distinguished, however, by the fact that chrysoprase 

 is harder and much less dense than prehnite. 



Chlorastrolite is a mineral which is sometimes referred to the species prehnite. 

 It occurs as small rounded masses with a finely radiate or stellate internal structure, and of 

 a bluish-green colour in the amygdaloidal trap rocks of the Isle Royale, Lake Superior, in 

 the State of Michigan. These rounded masses, in many cases, have been weathered from 

 the rock and are now collected as water-worn pebbles on the shores of the island. Owing 

 to their fibrous structure they exhibit a chatoyant appearance similar to, but less perfect 

 than, that of cat's-eye. When suitably cut they give gems of considerable beauty, which, 

 however, are worn solely in North America. The largest nodule hitherto found has a 

 diameter of 1 J inches, but the majority of specimens are much smaller. 



Zonochlorite is a mineral of very similar character ; it occurs in the amygdaloidal 

 volcanic rocks of Neepigon Bay on Lake Superior in Canada. The rounded masses, which 

 fill the amygdaloidal cavities, attain a diameter of 2 inches ; they are built up of alternate 

 layers of light green and of dark green material, and form rather pretty cut stones. The 

 mineral, however, is rarely cut except in America, and even there is but little used. 



THOMSONITE. 



The essential characters of a precious stone are not, as a rule, present in thomsonite, 

 but there is one variety of the mineral which is sometimes used for this purpose. This 

 occurs filling amygdaloidal cavities in volcanic rocks at Good Harbour Bay on Lake 

 Superior ; these rounded masses are sometimes weathered out of the rock and found 

 lying loose on the shore. Their internal structure shows a central mass of radially arranged 

 fibres enclosed by a few concentric bands which follow the outer boundaries of the mass. 

 In these concentric bands delicate shades of milk-white, yellow, and green alternate with 

 each other, so that a section of the mass has an appearance which resembles that of agate. 

 These water-worn pebbles, the largest of which are an inch in diameter, are collected on the 

 shore and polished ; no attempt is made to reach those still enclosed in the solid rock, and 

 stones which are worked are only worn in the place of their origin. 



Lintonite is a variety of thomsonite with alternate bands of green and of flesh-red ; 

 it is found in the above-mentioned locality. 



NATROLITE. 



Natrolite is another mineral belonging to the zeolite group ; like thomsonite, it is a 

 hydrated silicate of sodium and aluminium. It occurs sometimes as beautiful water-clear 

 crystals of elongated, prismatic habit ; these, however, have never been cut as gems. The 

 mineral is also found in masses built up of radially arranged fibres marked by concentric 

 bands of varied colours. Fine specimens of this variety of natrolite occur at Hohentwiel 

 in Wiirtemberg ; in these the concentric bands are coloured alternately isabel-yellow and 

 pale yellow or white. The mineral is susceptible of a good polish, and is sometimes cut in 

 such a way as to display the coloured bands to the best advantage. Even this variety of 

 natrolite can scarcely, however, be regarded as a precious stone, as it is of but little value, 

 and is used only for inlaying and in the manufacture of small ornamental objects of various 

 kinds. 



