Covering cement with gravel.

I'm told that there is a product which you use when putting gravel onto cement to make it look like a gravel path, but which won't move. The product is apparently applied to the gravel to prevent the cement sticking to it and making it look like a job done by an idiot.
But what's the product called?
Rob Graham
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.