Here are two examples of petrified wood - one natural and polished on the surface and the other cut and polished into a sphere.
Petrified wood has grounding earth energies, as well as wood energy, so in feng shui they would work well in both earth and wood areas of your home. You can also place them to nourish fire areas or metal areas.
Sectors where petrified wood would be auspicious include east (health), southeast (wealth and money), northeast (wisdom, spiritual growth, and education), or southwest (love and relationships). Place petrified wood in these locations with the intention to strengthen these aspects in your life.