The slightest pressure on a bevel, square or pencil edge which is in its original state of preservation can easilly damage it and cause it to fail the fingernail test for nicks. I see it often on MOP chips and plaques... I don't even own a micrometer and my rulers are plastic, where as a damaged ruler edge with a nick can easilly scratch Mother-of-Pearl just while holding the chip on the face of the ruler.