Hi there. I have had this stone for a little over a year now. Since then I have taken to gemologists at high end jewelry stores to be looked at, diamond buyers, and others in the trade and industry to help identify. Apsolutely no one know what stone it is, just that it is a natural stone. I have no way of finding out the RI of this stone and have literally no tools to do so either. My microscope was stolen, so I no longer have this either. I was able to look at through the microscope though a few times. It was difficult to focus because of the odd shape and large size though. It created halos around the stone which hurt my eyes so I didn't look at it for long.
I have confirmed it does have veil and fingerprint inclusions which are all grouped near the cutlet. There are some in other areas but this cut quite literally makes them disappear and reappear all depending on lighting conditions. It seems they help it reflect light at you because those areas of the stone can blind you when light is directed straight at the table. I posted the photo of this for you to see.
It also has a mirror type effect. At first glance you assume you are seeing through the stone. When examined, you are actually seeing the reflection of you and everything around you. This was confirmed the other day when I finally got a decent photo of my phone light inside the gem stone through the table. It was not even distorted, it was equivalent to a mirror image. at that moment I understood why it was so hard for me to previously capture focused pictures of it with a light source, it was reflecting the light right back at my lens. I posted the photo of this for you to see.
This stone has tested as a diamond before on two different diamond testers. The presidium diamond mate a and also a cheap one from the Internet. It doesn't do it all the time though, which is also weird. At first I thought maybe it was a colorless sapphire. Then I thought perhaps a colorless tourmaline. I also can't pin point the specific cut. It's mixed between a rose cut, briolette and a cushion.
If anyone can shed light on this it would be very helpfull. I'm out of ideas.