[Rockhounds] Re: Rockhounds Digest, Vol 43, Issue 18

[Rock Currier]

Peruvian opal vs. chalcedony.

Peru produces both gem silica and the more abundant blue opal. The opal 
often has black dendrites. The blue opal (Andean Opal) has been around for a 
long time, and the gem silica has been produced only during the last ten 
year or so. None of the gem silica was on the market until the last ten 
years or so, and even then, there was always a lot more blue opal than gem 
silica. One of my suppliers has spent a lot of time and energy trying to 
exploit the gem silica market and I think I have seen as much as any 
foreigner. I have classically used the luster on the fracture/cleavage to 
make the separation between the opal and the chalcedony, the opal having a 
shinier cleavage. I have also come to believe that there are materials in 
this class that may be a mixture of opal and silica and that there may be a 
sort of "solid solution" series that can take place between the Peruvian 
blue "Andean opal" and the gem silica. That is just an opinion of mine and 
not backed up by any mineralogical studies that I know of. Tim is also 
speaks practical truth when he suggests the difference is in the cutting. 
Most cutters don't have any mineralogical background and they class things 
by their color and how they cut. There are and always have been a lot more 
of them than mineralogists and their classifications and names for stones 
dominate the cut stone market much more than correct scientific names, and 
probably always will. One think for sure is that chalcedony is a rough tough 
customer compared to opal and as you point out, if it broke falling off the 
bed, it is probably opal. It would almost have to be opal. We sell a lot of 
real agate marbles and I often like to demonstrate the toughness of agate by 
taking one of them and throwing it down hard on the concrete floor of our 
warehouse and the marble will almost always bounce up four or five feet. I 
catch it and hand it to the customer to examine to see if they can find any 
damage or changes in the polished surface of the agate marble. You hardly 
ever see any changes at all. I tell them that you could never do that with a 
quartz ball. I call it the Currier bounce test for chalcedony.

Rock