During my work on diorama, the biggest problem for me was, how to make an realistic looking earth trench...
One day I was walking by a aquarium and pet shop, and I dropped by to buy some sand...But when I got into the shop something caught my eye...Something that can be used as earth...
The solution is this http://www.gohasties.com/photos/Lava-Ro ... a%20CU.JPG
Lava rocks for aquarium...You just need to buy brown type of this rock and to make dust out of big parts and you will have good looking earth...