I'm glad that I could add to your survival skill-set. I'd say this is the most valuable sciency bit that ambitious gem lovers need to know about. It shows itself in a lot of the tools and techniques we use, so once you "get it" it opens a lot of magical doors. Fear not! more edu-tainment on the horizon.

Glad I can help ~ yah, without being able to see stones when you hear these terms, its soooo hard to get the ideas to stick.

This is a fantastic question Tom. On the colour, there are some properties that are associated with colour, but birefringence is controlled by the mineral itself. Zircon is a weird exception because its RI and SG can change due to a group of radioactive elements in it. Once Zircon is heated (which much of the blue zircon is heated) it typically goes back to its high RI and specific gravity. I said it was weird... Other stones though, like corundum... ruby and sapphire... intense red ruby can have a sliiiightly higher RI than some sapphires, arguably because of the additional chromium content, but these might just be my crazy gem voodoo doctor theories on the WHY its higher. SO - regardless of the colour, there are stones that have larger and others that have smaller birefringence, and it typically is not related to the colour to my understanding.

@@GemologyforSchmucks Very interesting... and weird. Thanks!

The answer is... maybe with a loupe, but unlikely without one. An experienced observer may be able to see doubling using a loupe, but the birefringence of corundum isn't very large, which would make the doubling effect much more subtle than some other stones. Cheers~

@@GemologyforSchmucks thanks for the fast reply...very helpful

This is an interesting concept and would be worth an experiment with some highly doubly refractive stones for ease of experimentation. The one caveat that i'd bring up is that we dont necessarily know where the optic axis is on a cut stone, so its possible that you'll mark the stone on the optic axis and there will be no doubling in that direction.

@@GemologyforSchmucks Wooooaaaah! Mind blown! Fireworks! Thank you!🙏

It is simply a doubly refractive gemstone with colour change phenomenon. Alexandrite (colour change chrysoberyl) is one famous example. Others are singly refractive gemstone with colour change (such some garnets or spinels).

An excellent question ~ and one that only a gemologist near you who can put their hands on it, test, and inspect it, will be able to say reliably.

Pesky gemological fish