The hardnesses of several substances according to a scale known as the Knoop value are a...

The hardnesses of several substances according to a scale known as the Knoop value are as follows:

Which of the materials in this list would you classify as a ceramic? What were your criteria for making this classification? Does classification as a ceramic correlate with Knoop hardness? If you think it does, is hardness alone a sufficient criterion to determine whether a substance is a ceramic? Explain.