Though rare and specialist, this word for the scientific study of seals or signet rings has its place in the fields of decorative arts and archaeology.
Anyone who has acquired even a little knowledge of sphragistics knows that sometimes we find quite strange names on seals (only rarely attested elsewhere) or sometimes unique transliterations of foreign names — names not attested elsewhere in Greek.
Fifty Years of Prosopography: the later Roman Empire, Byzantium and Beyond, by Averil Cameron, 2003. Prosopography is the study of the public careers and relationships of a group in a particular place and period.
The original idea behind this word in English was very much tied up with diplomacy, since there was a real practical need to confirm that the seals on what purported to be official documents from foreign countries were genuine. So specialists studied the history of seals. They still do, but these days more because the study of old seals can reveal a great deal about the history of a period.
Sphragistics is almost the only word in English today that contains the /sfr/ combination of sounds, not the easiest cluster of consonants to say. It is from Greek sphragis, a seal (sphragistes is the Greek term for an Egyptian priest who kept and used the temple seal).
The field is also known as sigillography.