Bought 50 or so small stones at a local auction recently for study. Cost about $50.00. Those already identified by the seller in packets or containers proved to be as stated, according to my tests by basic small scale equipment and most of the others were also identified to my satisfaction by use of equipment and GT Pro database. Not certain as to natural or synthetic. There are a pair of small cabochons that are black to the eye at normal distance, dark red up close, pinkish red with chunky, irregular black inclusions that are more prominent in microscopic view. Flat bases are very black, making RI quite impossible for me to determine. SG appears to be 2.52. No indication of double refraction on polariscope or dichroscope. Many, many narrow black lines are seen clustered together under spectroscope. Any guesses? Unfortunately, I am not equipped to provide pictures.
A photo isn't enough for identification, but I'm having trouble envisioning the stones from your description. Even a phone camera shot would give some idea of what they look like (and, perhaps this is just my inexperience, but generally I think photos of cabochons can potentially convey more identifying information than photos of faceted stones).
Thanks to you both, for today's replies. My problem appears to have been difficulties with instruments. I have found that both my refractometer and specific gravity device can be quite finicky. I have now re-run the SG and found it to be closer to 2.36-2.40. This led me to obsidian on GT Pro software and picture of mahogany obsidian which looks pretty close to what I have. RI is given as 1.49. Rechecked on my refractometer and got about a 1.45 which agrees with Schumann's data. However, he shows density range of 2.35 to 2.60 which would cover the higher number I got at first. Seems identity has been discovered. Thanks again
Sorry, Barbra - do not have setup for posting pictures. However, if you have the GT Pro software for PC you can find a picture under "obsidian", sub "mahogany obsidian". Thanks for your interest.
assuming what you have is indeed glass, and assuming that you use the spectroscope away from fluorescent light sources (led, neon tubes) and away from day light (from the window), then a glass showing many narow absorption lines is most likely artificial glass.
However I suspect you've been using your spectroscope incorrectly
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