
ON QUARTZ WITH A PEARLY LUSTRE. 
BY 
RICHARD J. MOSS, rF.c.s., Keeper of the Minerals, R.D.S. 
[Read May 21st, 1877.] 
AT a former meeting I exhibited some specimens of crystallized 
quartz, with an unusual pearly lustre; since then I have care- 
fully examined these specimens, and others recently obtained, 
and finding that very erroneous opinions have been entertained 
concerning the nature of the lustrous coating, I propose to state 
briefly the results of my examination. 
The Rev. George Harrison Reade, who presented the specimens 
to the Society, states that he found them “imbedded in a ferru- 
ginous mud, in a fissure of a solid limestone rock, in the demesne 
of Rock Forrest, near Mallow, county Cork.” The largest specimen 
consists on one side of an irregular mass of ferruginous quartz, 
which has evidently been deposited on a bed of calcite, for there 
are impressions of crystals in the quartz of which I have taken 
casts, and found that the angles correspond with those of the 
more common rhombohedral forms of calcite. 
The hexagonal pyramidal crystals on the other side of the mass 
present no notable peculiarity, except in lustre. Instead of the 
usual vitreous appearance, they exhibit a pearly lustre, owing to 
a very perfect reflection of light. This lustre is not evenly dis- 
tributed, as occasional small patches on the faceswf the crystals 
exhibit the ncrmal lustre. On examination the pearly appearance 
is found to be confined to a coating, varying from an exceedingly 
thin film to a layer of about a millimetre in thickness. This 
coating can be detached in fragments from some of the faces 
without much difficulty ; the underlying surface then exhibits the 
usual vitreous lustre. The outer and inner surfaces of the 
detached layer are almost identical in appearance. On chemical 
examination I find that the coating consists only of silica, it does 
not contain any other substance in appreciable quantity. <A 
microscopical examination shows that the coating consists of 
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