The Greenstone Shepherds Whistle ( also called a Dog Whistle) is used in New Zealand by farmers, sheep herders and dog trailists.  And it makes an excellent Greenstone Pendant as well.

The greenstone whistle actually works, so you too will be able to enjoy using it.

Size: approximately 4.5cm (1 3/4") wide

How to blow the shepherd's whistle:

Hold the tab and place the whistle in your mouth.  The curved open edge will be facing out.  The curved solid edge will be inside your mouth, against your tongue.  Hold the whistle in your mouth so there is a gap underneath the whistle. (If you place your tongue against the lower hole, you will lose any sound).  Blow.  No need to blow hard, just blow across the top of the whistle, like you are blowing across the top of a bottle.  Once you have mastered that, you can also move your tongue up and down underneath the whistle, to create more sounds.