Truth

Ever heard the insulting remark that someone is "dumb as a rock"?
Think about this: Natural forces break stones down to become soil.
And God created Adam out of... what?

So, Jesus' statement was not an insult, but a statement of truth to
give perspective to the arrogant people standing in front of Him.

It is always good to keep things in perspective. If God sees value
in an inanimate stone, how much more does He value us?
  

Produce fruit in keeping with repentance. And do not begin to say to
  yourselves, 'We have Abraham as our father.' For I tell you that out
  of these stones God can raise up children for Abraham. (Luke 3:8 NIV)