The Jewel of Karnak is an emerald that belonged to Nefertiti.
The Jewel of Karnak was used as a prop in the filming of Pharaoh in 1936, but was lost when the set was bulldozed over.
When Nancy finds the set in 2008, she discovers the lost footage from the film and a note from Z. B. Collingwood saying that he had kept the jewel for himself and had filmed with a replica. Depending on the ending the player gets in Lights, Camera, Curses!, the footage either shows Lois Manson simply getting up from the floor and walking away, or opening her bracelet and hiding the jewel in a corner, which Nancy then finds.
Eda Brooks so desperately wanted the jewel that she followed Nancy to the old set. When she learned that there was no jewel (before the footage is watched), she tries to bury Nancy in an attempt to still become more famous.