What is the Greek root syllable 'petr-' in English?

The Greek root Word petr- means stone or rock. Some examples of use of this root include:

• Petrified - to be terrified; "turned to stone" (both literally and figuratively)
• Petracide - the destruction of stone buildings or monuments
• Petrography - the description and classification of rocks
• Petrichor - the smell after rainfall
• Petrobiont - lives among rocks