Caestus could be purely leather, and used for non-lethal "boxing" (as in their original form from Ancient Greece), studded with metal, or even fitted with iron plates for lethal damage in hand-to-hand combat.
From the word meaning "spheres", this consisted of leather strips or rings (hence the name) wrapped around the hands, with cutting blades or spikes protruding from between them, in order to cause bloody cuts from even a glancing blow.
With a name meaning "limb-piercer", this was a thick gauntlet or hand-wrap of tough leather, studded with iron or lead. A three-pronged "fork" made of bronze was attached to the glove for delivering potentially-lethal and piercing punches, thus giving the weapon its formidable name.
In the Roman arena, Caestus were mostly used when otherwise unarmed opponents - such as prisoners, criminals or slaves - were made to fight each other to the death.
They were also used outside the arena in violent competitions known as caestus boxing, until this increasingly brutal and bloody sport was officially banned in 393 AD.
In the episode The Thing in the Pit, Spartacus and Myrmex fight one another in The Pit using different forms of Caestus. Myrmex is issued sphairai, while Spartacus draws caestus (which here are a plated metal "brass-knuckle" style).