Virchow coined the name amyloid for a material he observed in tissues. This term means starch like, and is a reference to the earliest methods used to demonstrate its presence. The classical method to detect starch is to treat it with an iodine solution, when it turns blue. Amyloid initially stains deep brown with an iodine solution, but this turns blue after treatment with concentrated sulphuric acid. This is actually closer to cellulose than to starch, but at the time referencing an animal material as being the same as a plant material was not acceptable, so it was named starch like rather than cellulose like (celluloid, now used for another material derived from cellulose).
Amyloid is an acidophil, hyaline (glassy) material; that is, it is stained by acid dyes and is pale pink in an H & E, although its appearance is not sufficiently distinct to definitively identify it, and special stains are required. Most of these depend on a characteristic of amyloid that causes dyes to attach in a regular molecular pattern, forming a pseudocrystal. This word means false crystal, and describes the optical properties of amyloid that are similar to those shown by some crystals, and which are attributable to their very regular molecular structure.
Amyloid is largely protein, but it may contain other materials. For that reason sometimes it will be stained by methods for carbohydrates (alcian blue, PAS). In fact, this emphasises the point that amyloid is invariably identified in light microscopy according to its staining characteristics rather than its morphology. In electron microscopy it has a particular fibrillar ultrastructure which identifies it specifically, but this cannot be seen with a light microscope.
Wolman gave ten criteria for determining whether a material should be identified as amyloid or something else, adding that his Standard Toluidine Blue (STB) staining method should also be positive.
Variability of staining
There is some disagreement as to which staining methods are the most appropriate for determining whether a material is amyloid or something else. Drury and Wallington describe amyloid as being eosinophil, moderately PAS positive, variable with a Van Gieson and blue or green with a Masson type trichrome, disagreeing with Wolman, who said that amyloid is not stained like collagen (i.e. blue or green) in a trichrome. Others require different staining results for a positive identification, such as a positive staining with congo red in conjunction with the DMAB nitrite test for tryptophane, or crystal violet metachromasia in conjunction with green birefringence after congo red staining.
These differences of opinion as to what constitutes definitive staining results for amyloid are rooted in the inherent variability of the material itself. Many examples of amyloid will meet the stated criteria, but it is not by any means unusual for particular examples to stain according to a different pattern. For example, during the author's career he has had at least three examples in which the staining results caused confusion. In one, the congo red stain (Highman's) was positive, but sirius red (Llewellyn's) failed to stain it. In another the staining was reversed, with the congo red failing to stain with a positive sirius red. In the third, both congo red and sirius red were negative, as was crystal violet metachromasia, but Vassar and Culling's thioflavine T was weakly positive. The material was later confirmed to be amyloid by electron microscopy.
The point is that amyloid is a variable material and the staining may be variable as well. Particular examples of amyloid may stain with a particular method or not, and failure to stain may not mean that a particular material is definitively not amyloid. In practice, each laboratory should make available several staining methods, so that negative results which were expected to demonstrate amyloid can be checked with alternate procedures, or, as a last resort, sent for electron microscopy. I suggest either congo red or sirius red as the initial staining method, followed by whichever 