This mod does exactly what its name suggests; it adds armor and tools made from obsidian and emerald. Emerald armor and tools provide a mid-point between iron and diamond and Obsidian tools go just beyond the durability of diamond. And as an added bonus Obsidian armor provides you with fire resistance as well.
Emerald tools are slightly less durable than diamond tools but if you manage to obtain a great deal of obsidian, you’ll have much more success with those tools. Plus you can save your diamonds now rather than use them up creating tools and armor.