Buildings at the time were simple dwellings of one or two rooms, built from mud, stone and coral. Oil concessions in the 1920s and 1930s, and subsequent oil drilling in 1939, heralded the beginning of slow economic and social progress in the country. However, revenues were somewhat diminished due to the devaluation of pearl trade in the Persian Gulf brought on by the introduction of the cultured pearl and the Great Depression. The collapse of the pearl trade caused a significant population drop throughout the entire country. It was not until the 1950s and 1960s that the country saw significant monetary returns from oil drilling.