Custom post type specific item design

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’.