Gold, silver and cash

Q: In the following article on your website: http://alhaadi.org.za/articles/qurbani/668-who-is-required-to-perform-q… is mentioned that "Every adult Muslim, male or female, who owns 613.35 grams of silver or its equivalent in money, personal ornaments, stock-in-trade or any other form of wealth which is surplus to his basic needs, is under an obligation to offer a Qurbani."
Can you please give the reference of Hadith or other authentic books as to how the figure of 613.35 has been deduced?

Q: A young Mufti (scholar) said to calculate zakaat based on gold. Now he changed his ruling and says to give it based on silver. So, based on gold, the zakat was 3000$. Today when he told the people to do it based on silver, todays silver price was 500$. So what must the people do on paid zakaat, count what is paid on gold as okay and from here on pay of silver?

Q: Is zakaat fardh upon eighteen and twenty four carat gold?

Q: How will zakaat be paid on gold and silver? Similarly will one take the workmanship in consideration when calculating one's zakaat on gold and silver jewellery?

Q: What is the nisaab amount of gold and silver?