A finial or hip-knob is an element marking the top or end of some object, often formed to be a decorative feature.
Finial of the dome of the Taj Mahal
Stone finial at Aachen City Hall
Finials used to decorate the top ends of the rollers of a Torah scroll
Yemenite Torah case with finials
A pinnacle is an architectural element originally forming the cap or crown of a buttress or small turret, but afterwards used on parapets at the corners of towers and in many other situations. The pinnacle looks like a small spire. It was mainly used in Gothic architecture.
Pinnacles, studded with crockets, on King's College Chapel, Cambridge.
Pinnacles with statues on the roof of Milan Cathedral
Pinacles at Saintes Cathedral