Congo red positivity remains the gold standard for diagnosis of amyloidosis.
From the en.wikipedia.org
Congo red dye intercalates between the folds and, when observed under polarized light, causes birefringence.
From the en.wikipedia.org
Clinically, amyloid diseases are typically identified by a change in the fluorescence intensity of planar aromaticdyes such as thioflavin T or congo red.
From the en.wikipedia.org
Common to most cross-beta type structures they are generally identified by apple-green birefringence when stained with congo red and seen under polarized light.