Mystery Quartz Flat or Blank

Here's pictures of an 8-inch blank I acquired years ago that was claimed to be a quartz flat. Labels on the box suggest it's from the US Navy Norfolk Naval Shipyard. Does anyone know how to verify this is indeed a quartz flat and what it might be worth?

Thanks, Tom

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quick way is to measure its density fused quartz and fused silica have a density of 2.2 gm/cc or (137.4)(lb/ft3). Measure the diameter and thickness calculate the radius pi*r^2*thickness is the volume measure its mass divide the mass by the volume and you get density. Looks polished on both sides you can also measure the refractive index by measuring the thickness then measuring the optical path using a traveling microscope. Optical path length =refractive index*Thickness... Fused silica should have an index of about 1.46 at 500nm... I have used both methods many times to sort out mixed materials...