To my knowledge the oil industry was the first major modern industry, at least along the Arab Gulf (the story might be different in Western Arabia). Prior to the discovery of oil in the 1930s (Bahrain was the first in 1932), date farming and the pearl industry were two of the biggest and most important industries (the Kuwaiti emirs depended on great date farms for their wealth, and Gulf pearls were famous and highly lucrative business). Besides that there were smaller tradeable goods which the Gulf was well known for - for example, its high-class breeds of horses and donkeys.

The Gulf was most famous for its pearls, and though merchants were wealthy the business was risky and turbulent - it's crews were poor and in debt-bondage, and the job was difficult and hazardous to the health of the divers (due to having to dive deep underwater without protection from the pressures they has to withstand). Quantity of pearls varied year to year, while the market for luxury goods was dependant on the very wealthy. The business was completely undermined by the invention of cultured pearls in Japan in the 1920s, which made it cheap and reliable to produce pearls (and so drove the value of traditional pearl diving business to the ground). Coupled with the Great Depression and the shrunken market for such luxury goods, pearling industry was heading to obsoletion by the end of the decade. Good thing then that oil was discovered, first in Bahrain in 1932 and then across the Gulf over the next decades.