Definitely it was Sapphire & SteelSapphire & Steel
The opening credits include the narration that "All irregularities will be handled by the forces controlling each dimension. Transuranic heavy elements may not be used where there is life. Medium atomic weights are available: Gold, Lead, Copper, Jet, Diamond, Radium, Sapphire, Silver and Steel. Sapphire and Steel have been assigned." In the sixth and final assignment, Lead was replaced with Mercury in the opening credits.
The programme centres on a pair of interdimensional operatives, the eponymous Sapphire and Steel. Very little is revealed about their purposes or backgrounds in the course of the series but they appear to be engaged in guarding the continuing flow of time. They are two of multiple elements that assume human form and are sent to investigate strange events; others include Lead (Val Pringle), who takes the aspect of a jovial, friendly giant, and Silver (David Collings), a technician who can melt metals in his hands.
