In the process of developing a standard for research grade images, which are mostly covered in this video http://www.lep-net.org/?p=383 , Roy Larimer stated that the standard for digital imaging is 11-12% neutral gray. He made a batch of powder-coated aluminum plates f. Below is a suite of images of a plate he sent me (12in X 12 in) and a couple of shots taken with a Samsung 7 using LED lights. I do not have a high res image with studio lights, but these plates are absolutely wonderful. They provide a very nice smooth background and if they are cared for properly, they will last forever, you can use both sides of the plate.
I am selling the plates at cost for $60 as long as Roy makes them. I currently have two plates, one I will not cut and the other I plan to cut into four 6 X 6 in plates. I drill a small hole in the middle to put the pinned specimen. You can just putty was/putty or whatever you want on top of the plate instead of the hole.
You do not have to go with 11% gray background, as long as white balance is addressed. If you are fine with 18% gray card you can download this pdf file and make your own gray card. The quality of paper and printer will determine the quality of the background. Gray-Card-18-percent-RGB-128
You can also purchase gray cards from Amazon and several photographic sites.