On Thu, 25 Jun 2009 08:31:45 -0700, in rec.crafts.jewelry sunrainor
wrote:


Yes indeed it does - I sort of knew this already but you've described
it more clearly - I know that St. Lucia in my area (north) has
rhyolitic rocks, jasper for sure. It's not well known for lapidary
materials at all, but there's definitely a fair amount of these quartz
based materials.

Check these definitions. Last I heard, Rhyolite is an igneous rock. Though
silica rich, it is not jasper, nor related to it. Jasper, like other
chalcedonies, is a sedimentary material. Now, you could have rhyolites with
voids, bubbles, etc, form, which later would have jasper or other chalcedonies
deposit within those cavities. One classic example of that structure is
Mexican opal, which often is found filling holes and bubbles within a
rhyolite... But Jasper itself, is not generally referred to as rhyolitic I
don't think...


I'm happy with chalcedony for the translucent off-white-ish ones and
jasper for the yellows and reds. There are some that look like jasper
mixed in with lava - ie the dark parts have lots of bubbles and are
rough - these are much more brittle and I thought I'd call them jasper-
lava. Really I just don't want to claim anything that's not true -
like I read complaints about people calling non-agates, agate.

Again, I'd wonder just how lava and jasper could mix. This would require a
lava deposit below the water table, with sufficient porosity for mineral rich
solutions to penetrate the lava and deposit jasper within bubbles or voids. If
your material looks like lava has flowed into cracks or seams in the Jasper,
then I seriously doubt it's lava. Rather, it's likely something else that could
also deposit within cracks or fissures in the Jasper, at low temps, perhaps
hydrothermally. Hematite comes to mind...


I believe I will go to the library here and see if there's a rock ID
book and if not, I'll go ahead and get one like the one you
suggested...I do think someday I'd like to do some rockhounding trip
(s) and maybe I'll work with stones that aren't local also.

Beyond the basic gem identification book I suggested, which concentrates more on
classic gems, you may wish to be looking more at mineral guides, and see if you
can find some information on the basic geology of your area. If, for example,
you're in an area which does not have a volcanic origin, then you can be pretty
sure you're not finding lava. But this does not rule out other sorts of igneous
rocks (granite, rhyolite, Basalt, etc)

It might also be worth trying to see if a local high school or college has
anyone teaching geology or mineralogy classes. Those folks might be well
familiar with local geology and materials.

Peter

cheers
Finola