50	NATURAL HISTORY OF GEMS.
	Pliny notices (xxxvii. 22) that many were of opinion that this stone was of the same or a similar nature with the Emerald. This opinion has been proved correct by modern analysis, the component parts of each, in the same propor­tions, being Silica, Alumina, and Glucina, coloured by the oxide of Chrome. On these grounds, Phillips (Mineralogy: Beryl) states that " the only important difference between Emerald and Beryl is their colours; which, since they present an uninterrupted series, is altogether insufficient for a division of the present species." The Emerald is distinguished by its peculiar " emerald-green," which it derives fron; a small proportion of Chrome : all the varieties of other colours, tinged more or less, yellow, or blue, or altogether colourless, are Beryls. Yet, though thus iden­tical in their chemical constitution, the Beryl is by far the harder of the two, its hardness being denoted in the scale by 7-5 to 8, and therefore even superior to that of the Garnet. It appears also to be of a singularly compact texture, for antique intagli in Beryl are found retaining their original surface-polish more perfectly than those inj almost any other material. Kot so the Emerald. This gem held the same degree of rarity amongst the Romans as the true Smaragdus itself, for it was then obtained from India almost exclusively, " being rarely found elsewhere " (Plin ). Dionysius Periegetes enume­rates it amongst the gems gleaned by the Indian Ariani