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Identity HelpQuartz or opal?

5th Dec 2019 04:57 UTCMarie Wilson

I found this huge rock in the creek by our house after the seasons first rain. I thought it was quartz but saw some opal qualities in it and recently bought a book at a gem and mineral museum and the book opened up to a fire opal today on my passenger seat and now I’m confused. It’s really heavy and looks like it has clear bubbles in it. What do you guys think. I’m just starting out learning about this stuff but I had to drag it out of the creek before it was lost and I’m glad I did because the creek looks more like a river now since it’s been raining here for a few days now. Please help

5th Dec 2019 12:35 UTCcascaillou

For the record, opal is actually lighter than quartz (specific gravity 1.97 to 2.50 for opal, about 2.65 for quartz)

5th Dec 2019 14:18 UTCPaul Brandes Manager

Looks like either chalcedony or quartzite, I can't quite tell from the photo.

5th Dec 2019 14:54 UTCcascaillou

I think it's just quartz and feldspar, I guess the lustre and transluscency is likely only due to taking the picture wet.

5th Dec 2019 15:48 UTCKyle Beucke

It would probably help to break a piece off and use magnification to look at a fresh surface. 


5th Dec 2019 16:22 UTCAlan Pribula

Checking the hardness could also help.  Opal is softer than quartz.  If a known piece of quartz scratches your sample, it could be opal (but could also be a whole list of other minerals); if no scratch, then it's not opal and might be quartz (but could also be some mineral harder than quartz).

5th Dec 2019 16:41 UTCKevin Hean

Looks like dirty or iron stained Quartz to me, (milky quartz) 
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