The default template file for pages is page.php. If you use a static front page and you selected a "Posts Page" index.php is not used. from the codex :
* Front page - Select in the drop-down box the actual Page that you want displayed as your front page. If you do not select a choice here, then effectively your blog will show your posts on both the blog's front page and on the Posts page you specify. If you would like to create a static home page template file, do not name it home.php, otherwise you will encounter problems when you try to view the "blog"/"posts" section of your site. To get around this, just name it anything but home.php, for example, myhome.php
* Posts page - Select in the drop-down box the name of the Page that will now contain your Posts. If you do not select a Page here, your Posts will only be accessible via other navigation features such as category, calendar, or archive links. Even if the selected Page is Password protected, visitors will NOT be prompted for a password when viewing the Posts Page. Also, any Template assigned the Page will be ignored and the theme's index.php (or home.php if it exists) will control the display of the posts.
So if you do not select a "Posts page" in Settings/Reading, index.php is used