Hünefeld accidentally observed the first protein crystals— those of hemoglobin—in partically dried samples of mammalian blood blood pressed between glass plates. On page 160 Hünefeld noted that he had seen, in some samples, “tabular, crystalline precipitations, which under the microscope appeared sharply defined and colored bright red.” Figures 7 and 8 in the plate illustrating Hünefeld’s work show the crystals he observed in pig’s blood and human blood respectively; these represent the first published illustrations of hemoglobin crystals. Digital facsimile from the Bayerische Staatsbibliothek at this link.