Here's how to tie a Diamond Knot, also known as "The Sailor's Knife Lanyard Knot", "Bosun's Whistle Knot, and "Marlingspike Lanyard Knot".
This knot is based on a Carrick Bend and is tied with a single piece of rope. It make a really nice decorative knot at the end of a lanyard loop and can be used for some practical purposes as well.
You can find drawings and a description of this knot in Ashley's Book of Knots, knot number 787 for the single version and 788 for the double version (which this video depicts).
Enjoy!