Can you solve the puzzle of the missing diamonds?

Here's a puzzle from The Book of 500 Curious Puzzles:

A lady sent a diamond cross to a jeweler to be repaired. To provide against any of her diamonds being stolen, she had the precaution to count the number of diamonds, which she did in the following manner: She found the cross contained in length from A to B, nine diamonds; reckoning from B to C, or from B to D, she also counted nine. When the cross was returned, she found the number of diamonds thus counted precisely the same, yet two diamonds had been purloined. How was this managed?

Give up? Here's the answer.