<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !www.diamondx.com$ [NC]
RewriteCond %{HTTP_HOST} ^(www.)?([a-z0-9-]+).diamondx.com [NC]
RewriteRule (.*) index.php?category-name=%2 [NC,QSA]
</IfModule>