30 
NATURE STUDY. 
the corundum gems from all other precious stones of sim¬ 
ilar colors the word ‘ ‘ oriental ’ ’ is again used—oriental 
emerald, oriental sapphire, etc. Corundum with the tint 
of “pigeon’s blood’’ is the rarest and most treasured. 
Crystallization in these stones brings out the structure, 
and in a strong light a silvery six-rayed star may be seen, 
which gives the name “ star stones ’’ to these gems. 
All precious stones, except the diamond, are formed of 
compounds of different elements, and yet one stone may 
be identical with another in composition, but entirely dif¬ 
ferent in appearance and structure. The alexandrite and 
oriental chrysolite, for example, have the same chemical 
composition and yet they are totally unlike each other to 
look at, for the former is very dark-green by daylight and 
red by artificial light, while the latter is simply a greenish- 
yellow by both lights. Spinel is softer than corundum, 
but it ranges through an iris of coloring. Its red and rose- 
tinted varieties are the spinel and balas rubies, used as 
pivots for the wheels of watches. 
Quartz, when clear, is the Brazilian pebble of the spec¬ 
tacle-maker. Some quartz crystals seem of a very friendly 
disposition, and take into their hearts fine needles of other 
minerals. One quartz crystal exhibited has its “ghost’’ 
always on view. After the cry.stal was formed another 
started to grow in its interior and the result is truly ghost¬ 
ly. Other quartz cr 3 ^stals hold in their stony thraldom 
water and bubbles of air, and this, together with the fact 
that casts of shells are found consisting of pure quartz and 
chalcedony, tells that the mineral matter must have been 
deposited first in liquid form. From the smoky varieties 
of quartz we get the cairngorm and occidental topaz, 
whilst the violet amethyst is distinguished from all other 
varieties of quartz by its rippled fracture. Opal is formed 
from a combination of silica and water. Sometimes, how¬ 
ever, these two suFstances refuse to crystalize, and it 
