The Riddle of the Sphinx

I was recently reading the fourth installment of the Harry Potter series. As usual, when I got up to the Third Task, when Harry is in the maze and finds a Sphinx, J.K.'s utter brilliance got me thinking about making a Sphinx Riddle of my own. Until now, I have never really gone farther than just thinking about making a riddle. Now I have decided to take it one step farther and actually do it. I am finally going to fulfill my dream of creating my very own Sphinx Riddle. While I dont plan on it even coming close to the awesomeness of J.K.'s, I think you will all be quite impressed. I wish you all the best of luck, and just leave a comment with what you think will be the answer.

First think of an item found outside one's home,
Used to keep out dark, filthy loam.
Next think of a term used on the street,
For part of a car; neither engine nor seat.
Now think of an expression you commonly use,
When you get wind of some unexpected news.
Lastly think of a word much like knife knelt and known,
That is part of your body; in fact its a bone.

Now string these thing together to find what is said to be holy,
But as time goes on deteriorates slowly.