Review the original item:
As Irene scooped stuffing from the Thanksgiving turkey, gold glinted in
the breadcrumbs,
finally, the earring that she had lost early that morning reappeared.
You wanted to fix it this way:
As Irene scooped stuffing from the Thanksgiving turkey, gold glinted in
the breadcrumbs,
finally at last, the earring that she had lost early that morning
reappeared.
A comma splice occurs whenever you have two main clauses incorrectly joined with just a comma. Gold glinted the breadcrumbs is one main clause. Following it is finally at last, the earring that she had lost early that morning reappeared, another main clause.
To connect these two main clauses with just a comma between breadcrumbs and finally won't fix the problem, even if you toss in a couple of additional words like at last.
You might want to consult the rules for fixing comma splices and fused sentences.