I found this one in Maine this week, and just love it for for the intensity of its subject, looking a bit possessed and definitely as if he might see into one's soul (and with a soulfulness himself that makes me think of portraits of Lincoln.) The scratches and subtle halo around his head just amplifies the effect, as if he is radiating energy. The seller noted this as an amethyst ambrotype--definitely it is the latter, and though I have not taken it out of the foil frame it does seem to have a bit of a blue-purple tint so believe it is likely the former, too. 

3 3/8” x 2 13/16”. With degrading and scratching as evident.