TRK  TROPICA! 
AGRICCl /PURIST' 
[Oct.  r,  1896. 
273 
The  beautiful  specimens  of  rock  crystal,  calc  spar, 
topaZj  emer;i,lJ,  and  other  stones  svhicli  adorn  tniaeral 
collections  are  sufficient  evidence  of  these  propeities. 
But  it  is  very  certain  that  n.Uaral  crystals, 
although  they  possess  a beau'y  of  form  which  is  all 
their  own,  are  not  hy  c.  long  way  so  brilliant  as  thcfacet- 
ted  stoaes  which  are  c\it  fioiii  them  by  the  art  of 
the  lapidary;  that  a natural  diamond  is  not  so  lus- 
trous as  a facetted  brilliant. 
In  fact,  many  of  the  finest  gem  stones  present  a 
very  mean  and  sordid  aspect  before  they  have  passed 
through  the  hands -of  tiio  lapidary  ; one  has  only  to 
compare  the  dull  and  unattivactive  appearance  of  a 
parcel  of  rough  rubies,  sappKircs  or  diamonds,  with 
the  finished  jewels  displayed  in  the  Jeweller’s 
windows  to  see  how  much  these  owe  to  the  lapi- 
dary’s art. 
In  re-cuttiug  the  Koh-i-noor  it  was  thought 
advisable  to  spend  £8,00'J  on  the  process  aud 
to  reduce  its  weight  from  18li  to  lOll  carats. 
When  the  great  Pitt  diamond  was  cut  its 
weight  was  reduced  from  ill)  carats  to  HIT  ; 
and  tho  fragments  and  du.st  removed  were  valued  at 
£8,000;  but  the  extent  to  wnich  the  stone  was  im- 
proved is  indicated  in  the  fact  that  having  been 
purchased  for  £20,000,  it  was  afier  cutting  sold  for 
£13.5,000. 
To  understand  how  the  cutting  of  a precious  stone 
adds  to  ics  brilliaucy,  we  have  ouly  to  trace  the  course 
of  the  rays  within  ihe  stone,  and  consider  how  it 
cau  best  be  facetted  in  order  that  the  liglit  wiiich 
enters  in  v.irioin  diiections  on  the  upper  side,  or 
crown,  may  be  rellected  internally  from  facet  to  facet 
on  the  under  side  of  the  stjne  with  as  little  loss  a? 
possible,  aud  m ly  be  finally  thrown  out  from 
the  front  of  the  stone.  For  this  purpose  the 
facets  must  be  so  arranged  th.at  as  nuiih  of  the 
light  as  possible  within  the  crystal  shall  meet 
the  facets  at  an  inclination  exceeding  the  angle 
of  total  rettectim.  A brilliant  with  its  58  facets 
is  one  of  the  forms  which  experience  has  sliowii 
to  be  best  adapted  for  tho  purpo.se.  How  little  of 
the  light  ge  s througli  a stone  so  facetted,  and, 
therefore,  liow  much  of  it  is  totally  reflected  inter- 
nally, is  easily  shown  by  holding  tho  stone  in  a 
strong  beam  of  light ; first  so  that  the  light  is  re- 
flected, and  then  so  that  the  light  shall,  if  pos- 
sible, be  transmitted.  In  the  latter  case,  the  stone 
merely  throws  a dark  shadow,  indicating  that  little 
light,  if  any,  has  passed  through  it. 
A facetted  stone  is  always  cut  from  a single 
crystal,  and  not  from  an  ordinary  lump  of  the 
mineral,  which  is  generally  a mass  of  crystals. 
The  chief  reason  why  jewels  are  cut  from 
natural  crystals  is  that  the=e,  by  vir.ue  of 
their  crystalline  nature,  are  rejnarkably  homo- 
geneous, and  therefore  clear  and  limpid  when 
• tree  from  cracks  and  flaws.  A sto  ic  whicu  is  not 
homogeneous  can  never  have  the  purity  and  limpid 
brilliancy  of  a single  crystal,  for  at  evei-y  point  of 
contact  of  one  part  wiih  I’.nothor  reflexion  takes 
place.  Among  minerals  u.scd  as  precious  stones 
which  are  uot  crystals,  may  bo  laeiitioued  the  opal. 
The  opal  i^robably  owes  its  paculi  n-  beauty  to  the 
very  fact  that  it  is  filled  with  ntinntc  cracks  or 
cavities,  each  of  which  contributes  some  tint  of  colour 
by  reason  of  its  extreme  thinness,  just  as  the  colours 
of  a soap-bubble  are  due  to  the  thinness  of  its  film. 
Or  take  the  agate.  Here  the  stone  consists  of  layers 
of  diffevenc  materials  ditfereuliy  coloured.  Its  beauty 
is  of  a totally  different  nature  from  that  of  clear 
crystals,  .vhich  it  cau  never  rival  in  brilliancy. 
.Stones  like  the  agate  are  generally  olasiC  1 apart  as 
semi-precious  stones,  and  their  interest  depends 
upon  beauty  of  structure  or  colour,  or  pissibly  to  a 
large  extent  upon  their  rarity.  The  turquniie,  for 
example,  is  a very  rare  stone,  which  is  aiipareiitly 
absolutely  uiicrystallised,  liiit  possesses  great  beauty 
of  colour,  and  is  therefore  much  pri/.ed.  The  same 
is  true  of  cariielian.  On  tlie  present  occasion  we 
are  not  concerned  witii  those  opaipic  or  curiously 
structured  iiiiiierals  whose  beaut}'  resides  almost  solely 
ill  their  colour. 
Those  who  have  lial  no  practic,!,’  ac  I'liiutance 
with  minerals,  have  little  idea  how  xaiiablo  and 
accidental  are  their  coloiu’s.  They  may  scarcely  realise 
that  the  ruby  and  the  sai>phire  are  the  sainc  mineral, 
and  that  this  mineral  also  occurs,  and  is  used  in 
ewellerv,  ahsoliiU-ly  colourless  when  it  is  known  as 
ux  sapphir.^,  green  as  the  so-called  (Oriental  emerald 
and  yellow  as  the  so-called  Oriental  topaz,  that 
t ipaz  itself  may  lie  _\  el!ow,  brown,  blue,  or  colour- 
loss  ; tint  zircons  range  from  colour-less  through 
almost  all  conceivable  shtdes  of  brown  aud 
green,  and  that  even  diamond  has  been  fo  ind 
red  and  blue. 
When  we  came  to  consider  the  properties  by  which 
precious  et  nies  are  recognised,  I shall  say  little  or 
nothing  about  colour,  for  it  is  of  little  value  as  a 
criterion.  There  are,  for  example,  certain  red  stones 
which  the  most  skilful  experts  cannot  by  their  colour 
alone  refer  with  ceitaiiuy  to  ruby,  garnet  oi;  spinel. 
It  might  be  expected  that  a noteworthy  difference 
in  chemical  composbion  would  accompany  this  differ- 
ence of  colour,  or  that  the  pigment  could  be  ascer- 
tained by  analysis.  In  reality,  this  is  scarcely  ever 
the  case.  It  is  f.rirly  certain  that  the  emerald  owes 
its  colour  to  the  presence  of  chromium,  hut  the  vari- 
ation in  the  an.vlyses  of  precious  stones  cannot 
generally  be  attributed  to  anything  indicated  by  the 
variation  of  colour. 
The  chemical  composition,  though  of  groat  general 
importance  in  mineralogy,  is  of  little  pr.actical  v.ilue 
in  the  discrimiuatiou  of  precious  stones,  since  it  is 
usually  impossible  to  sacrifice  a sufficient  quantity 
for  chemical  analysis.  If  we  are  dealing  with  a 
facetted  stone,  not  even  ilie  smallest  portion  can  be 
utilised,  foi'  tear  of  injuiing  it. 
There  is,  however,  one  rem.xrkible  optical  property, 
which  is  ultim.itely  related  to  the  chemical  composi- 
tion. As  is  well-known,  many  substances  possess  the 
property  of  absorbing  certain  r lys  oi  light.  When  the 
solar  spectrum  pioduoe  1 by  a imitting  ordinaiy  daylight 
through  a slit,  and  transmitting  it  through  a prism, 
is  passed  through  tlie  glowing  vapour  of  certain  substan- 
ces, particular  rays  of  light  are  absorbed,  and  their 
absence  from  the  emerging  light  is  manifested  by 
corresponding  daik  bauds  in  the  spectrum.  The 
instrument  hy  which  tho  observ  itions  are  made  is 
the  spectroscope.  It  is  well-known  to  most  people 
that  the  solar  spectrum  itself  contains  certain  dark 
bauds  of  this  sort,  which  are  produced  by  vapours 
that  can  be  identified  by  tho  position  of  the  bands 
in  the  spectium  ; and  thus  it  is  possible  to  ascertain 
something  regarding  the  chemical  constitution  of  the 
sun  and  certain  of  the  lieaveuly  bodies.  Now,  a 
precisely  similar  effect  is  produced  by  certain 
elements  if  present  in  a mineral,  by  merely 
transmitting  the  light  through  a piece  of  it. 
Thus,  transparent  minerals  which  co.itain  the 
rare  element  didyiniu  n,  betray  the  presence  of 
tint  element  as  soon  as  they  are  viewed  through 
a spectroscop j by  ordinaiy  daylight;  the  appetrum 
is  seen  to  be  traversed  by  black  bands  in  the  green, 
which  are  qniie  clmraeteidstic. 
.\moug  gem  stones  there  arc  two  which  possess 
this  curious  property.  One  is  the  var.ety  of  red  gar- 
net, known  as  alraaudine,  aud  the  other  is  the  jar- 
gon. The  almandine  produces  characteristic  bands 
ill  the  green,  and  the  jargon  in  the  red,  green  and 
blue  portion  of  the  speotnnn.  To  see  these  remark- 
able ab.sorptiou  spectra,  to  which  attention  was  first 
called,  I think,  by  my  friend.  Professor  Church,  it 
is  not  necessary  to  look  through  tho  stone,  it  is 
quite  sufficient  to  place  it  iu  a strong  light,  and  look 
at  it  through  an  ordi  lary  pocket  spectroscope;  the 
light  which  enters  the  instruuieut  consists  largely 
of  rays  which  have  penetrated  the  stone,  and  been 
reflected  from  the  facets  at  the  back.  These  rays  pro- 
duce the  absorption  spectrum.  Iu  this  way 
we  are  enabled  to  identify  a jargoon  or  an 
almandine  merely  by  looking  at  it.  There  is 
no  te.st  so  simple  or  so  easy  of  application. 
It  is  curious  that  tlio  almuidine,  or  iron-aluminium 
garnet,  is  the  only  garnet  which  pre.sents  an  abs-irptive 
spectrum,  and  it  is  uot  yet  certain  to  what  element 
tho  bauds  are  duo.  In  the  case  of  jargoon,  they  are 
suppised  to  he  caused  by  the  presence  of  some 
uranium  compound  in  the  mineral.  All  the  almaudine 
garnets  which  I liavc  oxamiued,  aud  nearly  all  the 
jargoons,  show  tliese  characteristic  absorption  spectra. 
