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Gem Toughness


#1
   "Water is soft, yet look what it manages to carve out of the
much harder rock all around us."
Peter, other than the brut force of water moving rock, isn't it the
impurities and material being carried by the water that is actually
doing the carving and not the water itself? 

Well, now here’s my 2? worth. I agree in part with the above.
Firstly allow me to mention that in common usage,‘soft’ water is
relatively pure water; rain, distilled, deionised, etc. Which all
give a prodigious lather with soap. ‘Hard’ water is water containing
calcium and similar salts; calcium bicarbonate, sulphate, for
instance. Labs used to determine the ‘hardness’ of water by adding
standard soap solution, shaking well, and repeating until a lather was
obtained which lasted 5 minutes - I used to have to do it! The amount
of standard soap used was a measure of the calcium equivalent.

Next, comes the fact that as the atmosphere everywhere contains
carbon dioxide. Whenever it rains, or there is dew deposited, the
liquid running down the rocks is a solution of carbonic acid; a very
dilute solution, nonetheless strong enough to carve huge mountains of
carbonaceous materials (limestone, chalk,) etc into a honeycomb of
caves given long enough. It starts off as ‘soft’ rain water, wnd
emerges from the labrynth of tunnels as ‘hard’ calcium bicarbonate
bearing water.

I live in a place where this carving and tunnelling by rainwater has
been going on for many millions of years, and the top of nearby Takaka
Hill is a mass of elaborately fluted and convoluted marble and
limestone called ‘KARST’ country, which has to be seen to be believed.
At the base of that ‘hill’ (3000feet!) is a resurgence where
millions of gallons a day of beautiful cold, clean, crystal clear
water (12C, summer and winter) pours out of the mountainside. But
it is very ‘hard’! Hence the old clich? “water wears away stone” –
John Burgess; @John_Burgess2 of Mapua Nelson NZ


#2

Um, well, yeah, mostly. I’ll admit that this particular example was a
really poor illustration of hardness differences… Water’s erosion
of rock and the landscape around us usually is only partly due to the
type of hardness differences we refer to with mineral hardness. Some
erosion is due to the simple force transmitted by moving water to
exposed particles, some is due to actual scratching type hardness
(mineral hardness differences) between the eroded rock and particles
of various minerals (silt, sediment, whatever) carried by moving
water), some is due to chemical erosion, ie water’s effectivness as a
solvent, or as a chemical agent in conjunction with other impurieites
in it, such as “acid rain” being able to more effectively carve stones
like limestones, etc. All in all, it’s a complex action and a poor
example of “hardness”. Sorry about that.

Peter


#3

I would like to thank all who responded to my inquiry on gemstone
toughness. It was informative reading and I enjoyed the various
opinions and experiences of those who deal with this on a regular
basis.

“When we all think alike, no one thinks very much.” -Walter Lippmann

Charles Heick