COMMON OPAL. 65 
its position according to the direction in which they are 
held to the light. The countries in which they are 
chiefly found are Hungary and Iceland. 
They are sometimes set in rings ; and the prices at 
which they were formerly valued were, in the highest 
degree, unaccountable and absurd. At present their 
value is considerably lower, though they are still in 
great request as objects of curiosity. The phenomenon 
of their becoming transparent in water is supposed to 
be occasioned by that fluid soaking through their whole 
substance, in the same manner as the transparency of 
paper is occasioned by immersing it in oil. An hydro- 
phanous opal weighing 274 grains was kept four minutes 
in water, and, on being taken out, weighed 32| grains, 
having received in this short period an augmentation 
of five grains, or more than one sixth part of its whole 
weight. When taken from the water, these stones as 
they dry become again opaque. 
To preserve them in beauty and perfection, care 
should be taken not to immerse them in any but pure 
water, and to take them out as soon as they have ac- 
quired their full transparency. If these precautions be 
neglected, the pores will soon become filled with earthy 
particles : the stones will cease to exhibit their pe- 
culiar property, and will ever afterwards continue 
opaque. 
104. COMMON OPAL is a semi-transparent kind of 
opal, which does not exhibit any changeable refraction of 
colour. It is found in Germany, France, Italy, and 
other countries of the Continent, and is employed for 
brooches and other ornaments. A green-coloured 
Saxon variety is sometimes cut into ring-stones. 
105. MOTHER-OF-PEARL OPAL, or CACHOLONG, 
is a milk-white, yellowish, or greyish-'uchue kind of opal, 
which occurs in Iceland, Greenland, Spain, .and the 
island of Elba. It is sometimes cut into a concave 
form, for brooches, and other female ornaments. Ita- 
lian artists also use it for mosaic work. 
