Oct,  I,  1896.] 
THE  TROPICAL  AGRICULTURIST. 
277 
PRECIOUS  STONES. 
BY  PROF.  HENRY  A.  MtERS,  M.A.,  I’.R.S. 
From  the  earlieat  times  certain  minerals,  which 
are  conspicuons  for  their  beauty,  have  been  prized  for 
decorative  purposes  : the  brilliant  green  hue  of  mala- 
chite, the  deep  blue  of  lapis  lazuli,  and  the  rich  colour 
of  red  jasper  would  naturally  attract  early  attention. 
But  these  particular  minerals  are  not  numbered 
among  the  true  precious  stones  ; they  do  not  possess 
the  remarkable  qualities  which  endow  the  diamond, 
the  ruby,  or  the  topaz  with  their  peculiar  attrac- 
tiveness. The  two  essential  qualities,  namely,  bril- 
liancy and  hardness,  are  only  possessed  by  certain 
rare  minerals  ; a brilliancy  which  makes  them  un- 
rivalled for  ornamental  purposes,  and  a hardness 
which  protects  them  from  wear  and  tear,  and  makes 
them  practically  indestructible. 
It  is  difficult,  in  a town  like  London,  where 
every  jeweller’s  shop  is  ablaze  with  diamonds,  to 
realize  that  largo  and  good  stones  possessing 
these  qualities  are  so  rare ; that  thousands  of 
natives  are  toiling  in  the  river  beds  of  India, 
Burraah  and  Ceylon,  washing  out  from  the  gravel  or 
the  sand  the  little  blue  and  red  pebbles  which  are  to 
be  converted  by  the  lapidary’s  art  into  brilliant  jewels 
of  saphii-e  aud  ruby.  Even  in  that  wonderful  pit  at 
Kimberley,  where  half  the  diamo  ids  of  the  world 
seem  to  have  been  crowded  together  for  the  use  of 
man,  although,  perhaps,  ten  tons  of  diamonds,  worth 
more  than  T50,00i),  have  been  extracted  in  25  years, 
vet  those  which  weigh  more  thau  an  ounce  each  may 
be  counted  on  the  lingers. 
I,t  is  in  the  qualities  of  hardness  and  brilliancy 
that  such  minerals  as  malachite  and  lapis  lazuli  fail ; 
owing  to  their  comparative  softness,  they  would  not, 
if  cut  and  polished,  possess  the  sharp  edges  and  bril- 
liant surface  of  the  emerald  or  sapphire,  and  would  soon 
become  dull  and  rounded  by  friction,  oven  by 
the  friction  of  ordinary  dust.  Again,  since  they 
are  opaque,  they  can  never  flash  like  the  sap- 
phire or  the  emerald  ; and  yet  it  is  quite  a 
mistake  to  suppose  that  the  necessary  qualities  are 
confined  to  those  few  stones  which  are  familiar  to 
everyone,  such  as  the  diamond,  ruby,  sapphire, 
emerald,  garnet,  and  amethyst,  There  are  many 
others  though  they  are  not  so  well  known  I think  we 
may  fairly  assert  that  such  minerals  as  tourmaline, 
jaroou,  peridoie,  spinel  and  chrysoberyl,  though 
their  names  may  be  familiar,  are  not  stones  which 
would  be  recognised  by  any  but  those  who  are  in 
some  sense  experts ; while  other  minerals,  such  as 
sphene,  andalusite,  anixnite,  idocrase,  and  diopside 
are  possibly  almost  unkown  to  most  people  even 
by  the  reputation.  Yet  all  these  minerals  possess 
qualities  of  transparency,  hardness,  and  Deauty 
of  colour  which  render  them  extraordinarily 
interesting  and  attractive  as  precious  stones. 
(A  number  of  facetted  atones  cut  from  the  less 
known  mineral  were  thrown  upon  the  screen  by 
reflected  light.) 
Take  first  the  hardness.  A few  years  ago,  the 
hardness  of  stones  was  a very  important  character 
in  the  eyes  of  the  mineralogist  ; it  was  one  of  the 
characters  by  which  tiiey  were  invariably  identified, 
and  a distinguished  German  mineralogist  drew  up  a 
table  by  means  of  which  the  hardness  of  minerals 
can  be  compared.  Any  stone  is  said  to  be  harder 
than  the  minerals  of  this  scale  which  it  can  scratch, 
and  softer  than  those  by  which  it  can  be  scrtached. 
In  the  right  hand  column  the  gem-stones  are  ar- 
ranged according  to  their  hardness. 
MOHS’  SCALE  OF  HARDNESS. 
1.  Tale. 
2.  Gypsum. 
3.  Calcite. 
4.  Flour. 
5.  Apatite 
6.  Felspar 
j Sphene. 
’( Opal. 
I Diopside. 
I Moonstone. 
! Epidote. 
J Icocrase. 
I Peridote. 
VAxinite. 
7.  Quartz 
8.  Topaz 
9.  Corundum 
10  Diamond 
i Quartz. 
1 Tourmaline. 
' ’ 1 Cordierite. 
V Garnet. 
/ Andalusite. 
! Zircon. 
1 Emerald. 
VPhenacite. 
.Spinel. 
..  d Topaz. 
vChrysoberyl. 
j Ruby. 
* * ( Sapphire. 
•••  Diamond. 
Among  precious  stones  diamond  stands  out  pre- 
eminent as  the  hardest  of  all  known  substances. 
Ruby  and  sapphire  ar'e  scratched  by  diamond 
alone,  while  chrysoberyl,  topaz,  and  spinel  scratch 
all  the  rera.vining  stones,  although  they  do  them- 
selves yield  to  the  scratch  of  ruby  and  sapphire. 
The  hardness  is  a character  still  generally  utilised 
by  the  expert  when  he  is  in  doubt ; in  e.xperienced 
hands  it  has  some  value.  By  long  practice  it  is 
possible  to  form  a very  close  estimate  of  the  hardness 
of  a given  stone,  and  that  often,  not  by  scratch 
of  the  other  minerals  in  the  scale,  but  by  the 
feel  of  the  stone  against  a file  ; the  resist- 
ance offered  by  the  stone  to  the  file  is  taken 
as  a measure  of  its  hardnes.s.  It  is  not  a character 
capable  of  any  accurate  measurement,  neither 
is  it  to  be  recommended  for  use  by  inexperienced 
persons.  I hope  to  show,  as  I go  on,  that  we 
have  now  accurate  methods  of  testing  at  our  dis- 
posal which  render  the  trial  of  hardness  quite  un- 
necessai-y.  But,  none  the  less,  the  character  is 
one  of  great  importance  as  investing  the  stone  with 
durability  ; all  the  precious  stones,  except  moon- 
a;oue,  opal  sphene,  have  at  least  the  hardness 
of  quartz,  and  can  barely  be  scratched  by  metals, 
even  by  hard  steel. 
Take,  next,  the  quality  of  brilliancy.  This  de- 
pends upon  two  things — firstly,  the  manner  in 
which  rays  of  light  are  affected  when  they  enter 
or  leave  the  stone,  and,  secondly,  the  manner  in 
which  this  action  can  be  intensified  by  the  art  of 
the  lapidary. 
When  light  passes  from  one  transparent  sub- 
stance to  another  it  is  bent  or  refracted  as 
every  one  knows  from  the  bent  appearance  of 
a stick  plunged  into  water.  Consider,  now,  a 
ray  of  light  falling  upon  the  surface  of  a trans- 
parent stone;  a portion  of  the  light  is)  reflected,  but 
a portion  enters  the  stone.  In  passing  from  air 
into  the  stone,  it  is  refracted  inwards.  When,  on 
the  other  hand,  it  passes  from  a transparent  stone 
into  air,  its  course  is  reversed,  and  the  emerging 
ray  is  refracted  outwards  or  towards  the  surface. 
It  is,  however,  with  the  emerging  as  with  the  enter- 
ing light— the  beam  is  sub-divided,  only  a portion 
is  refracted  out,  another  portion  of  the  light  is  re- 
flected within  the  stone. 
It  is  very  clear,  then  that  of  the  light  travelling  in 
different  directions  within  a diamond,  a far  larger  pro- 
portion is  internally  reflected  than  is  the  case  with 
any  ocher  stone.  We  shall  see  presently  that  it  is 
this  property  which  gives  the  diamond  its  consummate 
brilliancy. 
Another  effect  produced  by  refraction  is,  as  every 
one  knows,  the  separation  of  ordinary  light  into  rays 
of  different  colours — it  is  seen  in  any  prism  of  glass. 
This  property  is  known  as  the  “ dispersion"  of  light; 
and  a stone  which  possesses  great  disnersion  will 
exhibit  a beautiful  play  of  spectral  colours — will  exhi- 
bit a high  degree  of  what  is  called  fire.  In  this  respect 
again  the  diamond  is  pre-eminent ; its  dispersion 
is  nearly  twice  as  great  as  that  of  other  stones. 
All  these  optical  properties  are  beautifully  shown 
by  those  un worked  jewels  of  which  the  smooth 
facets  have  been  produced  by  nature ; I mean  the 
crystals  of  the  various  minerals.  The  beauty  of 
natural  crystals  of  transparent  minerals  is  largely 
due  to  the  optical  effects  which  I have  just  beep 
describing. 
