sounds like it was a small piece of enamel or a portion of a fractured
cusp. Most likely this piece would have been removed anyway. I'd get
it checked but not worry about it. Your dentist has chosen to use
amalgam as a core build-up material. I do not use this material since
i believe there are better build up materials out there ones that bond
to the walls of the tooth and do not cause the tooth and gums to stain
black due to the corrosion of the silver. But it should be fine if all
you want to do is prevent the loss of your tooth. Hopefully you had
this discussion with your dentist, if not it is still considered by a
portion of the profession to be an acceptable material and philosophy
of practice.