I just cut a stone with the same ri and the same color change per sa. It’s clear almost I’ll post pics. It’s the synthetic cristinite. Don’t know if the stone you are talking about could be the same but it’s a very common synthetic in Australia and that part of the world. It’s actually an nice stone to cut and polish and has many unique properties. What caught my eye was the photos in the op. It looks exactly like it when it changes colors. Apparently I’m having brain fade this morning. It also fluoresces yellow orange under 365-395 light. I’ll post the pics under this message.
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I forgot to mention , this pendant is likely old from 1960 … thnx for help also, I will buy a polariscope and dichroscope and see what it brings me
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I would love to know if this has a maker’s mark on it. Sometimes, if it is modern, I am able to trace back an item through the seller. Also, that glow is very bright, I wonder if it is a synthetic gem.
Bob, thanks for posting that. I hadn’t heard of Christinite until now. Seems it is a type of silica glass. Does it have the characteristic inclusions found in other glass type simulants?
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Jeff, that amber U-glass is a very unique treasure! What an amazing piece of art and material! 
It’s unfortunately unclear
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Yes but extremely seldom. I enjoy it for experimenting with some cuts that are not clear in the instructions. I also enjoy the ri of 1.62. It allows me to play with something other than quartz designs without having to change angles of the original design. It’s very faceting friendly and inexpensive. Takes a great polish with cerium oxide or diamond. Comes in a bunch of colors and also color change types. Along with bicolor types. It’s easier to work with and less expensive than synthetic corundum or spinel. It’s great for test cuts. Lol. And things for my 6 year old granddaughter. Lol.
Definitely not cristinite. It was not produced until 2005.