I think there are many ways to do so. Here is a simple solution for the loop-template:
<?php
// set css class, if post-title is 'diamond'
$cpt_class = ('diamond' === strtolower( $post->post_title ) ) ? 'post-is-diamond' : '';
?>
<!-- Begin loop-template -->
<article <?php post_class( $cpt_class ); ?> id="post-<?php the_ID(); ?>">
... your stuff here ...
</article>
So only if the post-title is ‘diamond’, the article has the css class ‘post-is-diamond’.