[BroJoeK]

MrB: "Everybody SING now! 'Welllllll.... the fossils date the rocks and the rocks date the fossils and the fossils date the rocks, again!' "

Rocks are dated using:

• radiometrically -- decay rates of about two dozen different radioactive elements and
• and/or geological stratigraphy -- analysis of rock layering, including fossil-bearing layers.

Fossils are dated:

• radiometrically, if they include radioactive materials,
• and/or by reference to previously established ages of geological strata in which they were found,
• by reference to related fossils of previously established ages.

Of course, some of these methods are "derivative" of others, and each has associated difficulties.
But the "gold standard" is radiometric dating, and when several different materials agree, then that's as good as it gets.

[MrB]

and when IF several different materials agree

which doesn't happen as often as you imply.

What happens is that 3-4 methods are used, the results are wildly different, and so the person tests it asks "what fossils do you see in the layers above and below this sample?", and the "indexing fossils" are used to determine which of the radiometric tests to use.