The index.html file name offers no value for SEO nor users. I recommend not using it and directing users to a "clean" URL (eg seomoz.org/ instead of seomoz.org/index.html).

As long as you properly set this up and consistently link to the same URL there are not any duplicate content issues. If you wish to go one step further you can add the canonical tag to your page.

What you are asking about is relevancy. If you write an article on the topic of "old silver coins", if your page is perfectly optimized for that key phrase then your url, title, links, content will all use that phrase. Your page is still relevant for "silver coins" but all the pages which are optimized for "silver coins" will beat you assuming the other factors are equal.

When you perform a search for "silver coins" on Google.com, over 13 million results are returned. 90% of people do not look past the first page so the reality is you are competing for the top spots on the first page. If your result isn't on the first page, it doesn't really matter much.

The bottom line, yes your page would be relevant for "silver coins" but most likely it wont be relevant enough to make any meaningful difference.