Loomi are also known as Omani Limes and Black Limes are common in the Arabian Gulf. Their tart lemon flavor is made by boiling them and afterward drying them in the sun. They can come in black and brown colors. 
 
In the Gulf of Arabia, it's popular to use them in slow-cooked stews and soups. To use them properly just pierce a few holes and throw them into whatever you are cooking. The longer and slower they cook the better!  
 
Origin - Oman