CATALOGUE OF GEMS. 



54a 



III. — Comparatirr tahJcit of tin 



ilorx tnid (HxtuujninJi'nHj cJniradrrs (if tlir lirllfr-liioirn 

 gems — Continued. 



XL— GEMS POSSESSING .V 



PLAY OF COLOR OR EXHIBITINii 

 OPALESCENT EFFECT. 



A THATOYAXT OR 



[No attention is here paid to the grounri or body color of the gem, but only to the play of ('olor, 

 chatoyancy. and opale.'ioenee. These efYeets may l)e of several kinds. In some, as in the opal and 

 labradorile, there may be a variety of hues of eolor, which change as the gem is viewed fnrni differ- 

 ent positions; in others there may be a reflection having the appearance of a white six-rayed star 

 on the body or ground color of the gem, as in the .sapphire; or the reflection may have the appear- 

 ance of a point or mass of pearly light, which .sometimes appear.s to occupy the whole of the stone, 

 and which varies according to the inclination given to the gem. The moonstone is an example of 

 this clas.s.] 



(a) Gems having x play of color or iridescence. 



Specific 

 gravity. 



Refraction and 



refractive 



index. 



Disper- 

 sion. 



Opal. 



Labradorite 



1.9 -2.3 

 Common- 

 ly 2.1. 



2.72 



5.5-6.5. Varies, 

 but in general 

 will scratch 

 glass slightly. 



6. Scratches glass 

 slightly. 



Oligoclase (heli- 

 olite sunstone. ) 

 Albite (peristcr- 

 ite). 



.do . 



.do , 



(b) Gems having star-like reflections, asteria. 



Corundum (star 

 ruby, star sap- 

 phire, asteria). 



Garnet 



Tourmaline 



3.5-4.3 

 2. 99-3. 22 



9 Double ... 



6.5-7.5 Single . 



7-7.5 Double, 



(c) Gems having chatoyant reflections. 



C h r y s o b e r y 1 



(cymophane). 



Beryl 



Quartz (cat's-eye) 

 Ort hoclase 



(moonstone) . 

 Albite (moon- 

 stone). 



2. 73-2. 76 

 2. 35-2. 7 

 2.4 -2.6 



8.5 . . . 

 7.5-S . 



6 



6 



Double 

 do. 



XII.— GEMS HAVING A NACREOUS LUSTER. 



