The emerald and the aquamarine are included by mineralogists under the species beryl· The differences of colour are due to minute traces of compounds too small to be determined with exactitude. The chemical constitution and the crystalline form of all the varieties of the mineral are the same. The form is a regular six-sided prism, belonging to the hexagonal (rhombohedral) system. This prism is often striated, both internally and externally, with delicate lines and fissures which are invariably parallel with its sides, not, as is the case with quartz crystal, at right angles with or across these sides.
The specific gravity of the different varieties of beryl, when free from flaws, cavities, and intruding minerals, is as nearly as possible 2.7. So the emerald and the aquamarine are a little heavier than rock crystal (2'66) and much lighter than green garnets (3'85) or green sapphires (4). Here are a few determinations of specific gravity made with stones of different colours :
The hardness of beryl varies between 7-5 and 8, the fine emeralds of Muzo being less hard than the aquamarines of Brazil and Siberia ; they are also rather brittle. The indices of refraction for the green ray are in the emerald of Muzo :
The dichroism of some forms of beryl is very strong ; luis is particularly the case with the emerald. Viewed across the prism with the dichroiscope the two images of the emerald are seen to be of different hues of green—one verging on yellowish green, and the other being a green with a tinge of blue.* The same
* Fig. 3. Frontispiece.