Vol. 5] 



Louderback. — Benitoite. 



353 



has pointed oat, the entire lack of such a compound, consider- 

 ing the frequent association of the oxides in the same formation, 

 is striking. 



Aside from these general considerations bearing on its chem- 

 ical character, the symmetry and physical constants of benitoite 

 do not appear to the writer to indicate any close relation to 

 beryl. 



Benitoite as a Gem. 



Benitoite forms a beautiful gem stone, but only a small pro- 

 portion of the crystals are suitable for cutting. It is generally 

 cut as a brilliant to bring out both its color and the brilliancy 

 and lire due to its high refractive power. The deep violet-tinted 

 blue of the finer stones is very attractive and by many it is 

 considered more beautiful than that of the sapphire, which it 

 distinctly surpasses in fire and brilliancy. The color varies 

 from colorless to very deep blue, the two extremes being the 

 least common. The finest stones are those with a moderately 

 deep color. Considerable judgment has to be used in getting 

 the proper attitude and proportions to bring out the full color- 

 value while preserving the brilliancy. Both color and brilliancy 

 vary materially in different directions and the color is often 

 distributed unevenly or zonally. Quite a number of stones which 

 have been cut by various lapidaries do scant justice to the possi- 

 bilities of the gem. A common result is the production of a 

 sort of dull leaden, or in the darker varieties blackish appearance. 



The large stones have a general run from a carat and a half 

 to about two carats, only a few exceeding the latter value. The 

 largest flawless gem yet obtained weighed a little over seven and 

 a half carats when first cut, though it has since been repolished 

 and brought down to a little below this value. It has a moder- 

 ately deep blue color and is about 14.5 mm. long, 10.5 mm. wide 

 at the girdle, and 8 mm. deep. It is the property of Mr. G. 

 Eacret of San Francisco. This stone is remarkable in that it is 

 about three times as heavy as the next largest flawless stone so 

 far obtained, and also that it was found in the early days of the 

 mine almost at the surface. It is shown actual size in plate 33, 

 figures 1-4. 



