Hessonite is a variety of grossular garnet, and garnet as a whole is a group rather than a species: a family of silicates sharing one structure in which different metals occupy the same site. Almandine, pyrope, spessartine, grossular, andradite and uvarovite are the principal members, and most natural garnets are mixtures of two or more.
Hessonite is grossular coloured by iron and manganese to a brownish orange, and it has a characteristic internal texture, a swirling granularity often described as a heat haze or treacle, which is diagnostic enough to identify the stone by eye once it has been seen.
Garnet's greatest historical moment was not as a cut gem. Across the fifth to seventh centuries, from the Black Sea to the North Sea, the prestige metalwork of the migration period was cloisonné garnet: thin plates of red garnet cut to fit cells of gold, backed with patterned foil to throw light forward. The Sutton Hoo shoulder clasps and purse lid are the best known examples in Britain, and the same technique appears in Merovingian, Ostrogothic and Central Asian work.
The garnets themselves travelled a long way to get there. Analytical work on the composition of the stones has traced a substantial proportion of them to South Asian sources, which makes a set of Anglo-Saxon jewellery evidence of trade running the width of Eurasia. When the supply changed in the seventh century, the style changed with it.