pratt] CORUNDUM GEMS. 53 
While the color of these sapphires is not as dark as the highly-prized 
Ceylon stones, they show a richness and brilliancy not equalled by 1 he 
oriental stone. They not only show a strong rich color by transmitted 
light, but their color is almost as good by reflected light. Then 
again, while many blue sapphires make beautiful day stones but are 
dull at night, the Yogo sapphire is a very brilliant nighl as well as 
day stone. 
The crystallography of these sapphires is markedly different from 
that of the sapphires of the Missouri bars and Hock Creek (page 50). 
The latter all show a prismatic development, while the former are all 
rhombohedral crystals, uone of which show the presence of any prism 
face. 1 
The crystals are etched and striated to such a degree that no crys- 
tallographic measurements on the reflecting goniometer w^ere possi- 
ble; but sufficiently accurate angles were obtained with the contact 
goniometer to permit the identification of the laces. The only two 
faces that could be identified were the base c (0001) and the rhombo- 
hedron x (3032) which is a new face for corundum. On one crystal 
two very small faces were observed which were too small to be meas- 
ured with the contact goniometer, but were probably the faces of a 
pyramid of the second order. 
In determining the rhombohedron, ten or more independent meas- 
urements were made of c/\x. These varied from 66 to 68°, but 
approximated closely to 67°, which agrees very well with t he calculated 
value, (17 :;', for 0001 A3032. These crystals are represented on PL V. 
The crystals are developed, as shown in tigs. 1, 2, and 3 of PI. V, 
the prevailing type being Like fig. 3 of PI. V. The crystals vary from 
those where the base is x^vy largely developed, having a diameter of 
8 mm., while the rhombohedron is only 1 mm., to those that have the 
base and the rhombohedron equally developed (fig. 1 of PI. V). Where 
the faces are more equally developed, the rhombohedral faces are 
generally rounded. 
The basal plane often shows characteristic striations which are par- 
allel to the three intersections of the base c and the rhombohedron 
,r, as shown in fig. 1 of PI. Y. These lines are sharp and distinct and 
on the very flat crystals can easily be measured when examined under 
the microscope. The rhombohedral faces are very roughly striated 
without showing any distinct parallel lines. 
One very common development of these crystals is a repeated growth 
on the basal plane of the rhombohedron x (3032) and the base c (0001), 
as represented in fig. 2 of PI. V. These growths are exceedingly 
varied, as is shown in figs. 11-14 of PI. V, where they are drawn 
in basal projection. In fig. 11 of PL V there is but one secondary 
rhombohedron and base, which has one of its rhombohedron faces 
a continuation of one of the rhombohedron faces of the crystal. 
1 Am. Jour. Sci., 4tli series, Vol. IV, 1897, p. 424. 
