Both mantra are fine. With “Somay Namah” you are including moreof Shiv tatva. Which would be more effective. When you chant this mantra have a om binural beat in background that would produce even higher vibrations. Something I could find on Youtube… https://www.youtube.com/watch?v=jKf5H9DhmX4
chant loud
I know of a site where you could fine the original stones (based on my experience). Though they charge more. http://venusjewellers.com/
Mars controls fire element and have power to free from past bad deeds. moonga should be used in Gold. it can be put in silver as well but gold is best.
Om Namah Shivay.