I definitely doubt that the back glass is BMG. If it is at all translucent, then I do not see how it could be BMG. BMG is closer to the color and radio transparency of metal than it is glass. However, I can see the use of BMG as a support for the from oled. This would be just a good too me. This would mean that there is a significant about of BMG in the iPhone and that it truly has a unique application that nothing else could achieve. I am not a hardware product design engineer, but I can see the strength of BMG ver instrumental in keep the from display panel thin and durable and able to support the force touch feature of the iphone.
Have a listen to the first Minute of this hands on Video with the iPhone X. They discuss the back panel, its shade, finger print resistance, how the glass 'wraps' seamlessly into the Stainless Steel and how it feel in the hand. I see a reflective Mirror surface, i'm not sure about translucency.
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