42 
THE NATURAL SCIENCE JOURNAL. 
the surface of the ground, in the Auck¬ 
land district. Now there are perhaps 
4,000 whites and 1,000 Maori engaged 
in this industry in that province, where 
alone the gum occurs, and from which it 
is largely exported to Pmrope and Amer¬ 
ica for making varnish. 
The copal resin, like amber, is rich in 
included insects, and the Chinese in a 
very clever manner insert brilliant col¬ 
ored beetles in copal and kauri gum by 
heating, and sell the product extensively 
as being natural inclusions under the 
pretense that the beetle is of the same 
age as the gum in which they have put it. 
THE OPAL 
Robekt Burnham. 
T is not my purpose to give you a 
scientific article on the opal in all 
its varities, nor to tell you why one or 
two substances containing from 90 per 
cent, to 95 per cent, of silica, with more 
or less water and metallic oxides and 
traces of earthy matter, etc., too numer¬ 
ous to mention, should be iridescent— 
having a play of colors—and be a pre¬ 
cious or fire opal, while the other sub¬ 
stance, chemically the same, according 
to the most delicate tests, should be 
plain as a piece of glass, with no irides¬ 
cence, a mere common opal or opal 
agate. It is easy to suggest a reason 
for the cause of this “ fire ’’ as we call it, 
and which is as good a name as any, be¬ 
ing easy both to remember and to under¬ 
stand ; and it is easier to admit such 
reason than to prove it incorrect. 
Dana tells you what someone else says 
about it, but of himself says as little as 
possible. I was engaged for some time 
in the opal trade at Queretaro, Mexico, 
and saw the stones there in all con¬ 
ditions. 1 went to the mines and bought 
them in the rough, and had them cut by 
native workmen, so while I do not go 
into scientific details, I can tell some¬ 
thing of practical interest. 
Queretaro is the center of the opal 
trade of Mexico, and while nearly all 
the opals are cut and sold in the city, 
the mines are from 20 to 40 miles dis¬ 
tant. 
The whole country is a mass of vol¬ 
canic rock, and wherever water has re¬ 
moved the scanty soil, hardly more than 
a few inches deep, there you see ex¬ 
posed the solid lava, hardly cold. 
There is no doubt in my mind that the 
opal and some other precious stones—if 
not all of them—are of volcanic origin, 
or result from volcanic action. 
In some cases you will find masses of 
opal stratified and evidently of sediment¬ 
ary or water formation. The mineral 
water evaporating left stratas of different 
colors, among others the opal. This is 
my idea of the formation of the opal; 
you have it for what it is worth. 
Take tlie finest gem and a clear, color¬ 
less stone found in the same matrix, re¬ 
duce both to powder and the chemist 
cannot tell “ t’other from which.” 
There is no accepted theory concern¬ 
ing the “fire” of an opal; no one is 
sure of its cause. Some say it is caused 
by small fractures, the sides of which 
reflect one upon the other. Others assert 
that it is the effect of enclosed water; 
some specimens have been said to con¬ 
tain 10 per cent, or more. 
The mystery about it only renders the 
gem more attractive; its beauty is 
granted. It is the only gem that has 
never been successfully imitated; the 
