colour                               35
	as does a prism, and show a wonderful exhibition of prismatic colour, which is technically known as a " play of colour." The descriptive term " opalescence " is self-suggesting as to its origin, which is the " noble" or " precious" opal; this radiates brilliant and rapidly changing iridescent reflections of blue, green, yellow and red, all blending with, and coming out of, a curious silky and milky whiteness, which is altogether characteristic. The moonstone is another example of this peculiar feature which is possessed in a more or less degree by all the stones in the class of pellucid jewels, but no stone or gem can in any way even rival the curious mixture of opaque­ness, translucency, silkiness, milkiuess, fire, and the stead­fast changeable and prismatic brilliance of colour of the precious opal. The other six varieties of opal are much inferior in their strange mixture of these anomalies of light and colour. Given in order of value, we have as the second, the " fire " opal with a red reflection, and, as a rule, that only. The third in value is the " common " opal, with the colours of green, red, white and yellow, but this is easily distinguishable from the " noble " or " precious" variety in that the common opal does not possess that wonderful " play " of colour. The fourth variety is called the " semi-opal," which is really like the third variety, the "common," but of a poorer quality and more opaque. The fifth variety in order of value, is that known as the "hydrophane," which has an interesting characteristic in becoming transparent when immersed in water, and only then. The sixth is the " hyalite," which has but a glassy or vitreous lustre, and is found almost exclusively in the form of globules, or clusters of globules,