In Descent, can you make an attack without an implement?
Yes, a character can attack unarmed.  "If a hero attacks without a weapon, he attacks with his fists.  This is a melee attack that grants one red die and has no special abilities."  (Descent rules, p19.)  So it's a base melee attack using the minimum melee dice - the red plus however many black.

Ranged and Magic attacks cannot be made without a weapon.  

If you want to house-rule allowing weaponless attacks of all three kinds, I would argue strongly in favour of using the base red (melee), blue (range) or white (magic) die as well as the black; this is the basic die used for all weapons of that type and is important for game design in distinguishing the range/damage properties of each attack type.  These basic attack dice also carry the important one-in-six automatic-miss chance.