"One of the masked men grabbed the glass necklace," went on Mr. Van Swigget. "He fingered it, cursed, and threw it to the floor. At gunpoint the pair forced me back upstairs. I had to open the safe and give them the diamond necklace."
--- ENCYCLOPEDIA BROWN FINDS THE CLUES by Donald J. Sobel, page 69, paragraph 4.
This is the last paragraph I read.
(Sadly, I couldn't solve the mystery without looking in the back of the book.)