The Heroes of Tolmurr
Arcane Casters are not necessarily Thumancers, and Thumancers are not necessarily Arcane Casters. The relationship between an Arcane Caster and a Thumancer is equivalent to the relationship between a Blacksmith and a Swordsman. Thumancers know the secret art of creating magical foci from active Vis Crystals; an Arcane Caster understands the movements, the rituals, and the phrases required to mold the magic that is innately imbued within the crystal. Without a foci, however, an Arcane Caster is as mundane and devoid of magic as anyone else.
Bards, Rangers, Wizards, Eldritch Knights and Arcane Tricksters cannot cast spells without the use of a Vis Crystal foci. Most casters have a smaller Vis shard on a necklace to allow them to cast cantrips. For anything more powerful, they require a Vis Crystal focus. Bards tend to have them set into their instruments, while Wizards tend to use staves and wands. A Ranger, Eldritch Knight or an Acane trickster may have a pure piece of crystal or may set their weapons with the Crystals to allow their spell casting.
For an Arcane Caster spell casting components do not take the form of a piece of lint or a feather, but rather a specially crafted Vis shard, or a specific element of liquefied Vis Essence.