The Belomo 10X 21mm triplet loupe magnifier has a large viewing area - 19mm.
I have searched for a good quality loupe to offer you (and for myself). I bought a whole bunch of them and tried them all out. I found the Belomo one to be the best. I even found it to be better than a more expensive "brand" name loupe, which is known for it's high quality. The Belomo is a "true" triplet loupe with glass optics and a metal construction. There is a large viewing area making it easier and more comfortable to use. It offers a clear sharp view of the inner depths of your crystals. These loupes are made in Belarus, a former state of the USSR by a company who once made high quality optics for military use, including high resolution satellite cameras.
It is a high quality instrument for an extremely reasonable price.