Get some vinegar. Drop some on the rocks and gravel. If it fizzes dont use it. If it dosent got to step 2, pH. Get a bucket and let it sit overnight with some tap water. Take the pH in the morning, and add the rocks. Then take the pH again the next day. If there is a big change dont use them.
For driftwood, make sure that it is dead. Make sure that there is no bugs that can rot. Then you need to out it in a bucket with some water, untill it will sink. to speed this up boil it.