360 THE DIAMOND AND OTHER PRECIOUS STONES. 



character only with the tourmaline. This latter stone, of \Yhich we shall 

 say ver^ .ittle as a gem, is highly prized in optics on acconnt of its polar- 

 izing qualities, which are utilized in a great variety of apparatus. It is 

 without any brilliant luster ; and though proposed as a stone for mourn- 

 ing ornaments, to compete with jet, jewelers have not yet made up 

 their minds to employ it for this purpose. For a really rich mouruing- 

 decoration, black diamonds are the gems to be used, as thej^ have been 

 in Portugal in decorating the crown-royal. The earliest specimens of 

 tourmalines came from Ceylon through Holland, The red tourmaline 

 of Siberia, called also siberite, is pretty enough for a ring ; it occurs 

 in minute crystals. The amateur of whom I have spoken had in his 

 collection very small siheritcs, from Corsica, of a crystalline form and 

 exquisite in color; they vrould have served as gems for the decoration 

 of the Liliputians. There are beautiful green and blue tourmalines, 

 which come from South America, and are called Brazilian emeralds and 

 sapphires. The aqua-marine, the name of which indicates its sea-green 

 hue, is a stone of a mineralogical character similar to that of the eme- 

 rald, but little in demand at the present time. It is possible that there 

 may be an augmentation in its price, since no new ones are received in 

 market. This stone loses nothing of its appearance in artificial light, 

 and it is sometimes curious to see a magnificent decoration of sapphires 

 wanting in effect at night, while a cheaper one of aqua-marine is not 

 only preserving its splendor, but seeming to gain in brilliance by can- 

 dle-light. The English regard aqua-marine with the partiality the Span- 

 ish had for the topaz. 



This stone takes a beautiful polish, and preserves it for a long time. 

 It is less hard than the topaz, and possesses many optical qualities, on 

 which our limits will not permit us to touch. 



We come next to the amcthysfj the name of which signifies a specific 

 against dninlcenness. It is a true rock-crystal of a beautiful violet color ; 

 it is essentially a daylight-stone. Nothing is wanting to this lovely 

 gem but rarity. Pliny employs the word amethystize as synonymous 

 with violet ize. Modern savans, with their lynx eyes, find a difference 

 between violet rock-crystal and pure amethyst. The latter is charac- 

 terized by a series of little undulated strata, which is wanting in the 

 violet rock-crystal. There are specimens of colorless quartz which 

 have a structure similar to that of the amethyst. When certain agates 

 consist of very thin layers of a uniform thickness, they take the colors 

 of the spectrum, and are called iridescent agates. It is probable that 

 the myrrhine vases, whose value reaches some hundreds of thousands of 

 francs, were cut from iridescent agates. Sir David Brewster has given 

 the exact theorj^ of this iridescence, ignorant that I had already done 

 so before him in the reports of the Institute. 



The same philosopher has also demonstrated that the color of sea- 

 shells is also duo to their surface being striated by undulating and 

 closely approximated minute lines ; for, if we take the impressions of 



