The small desert state of Qatar is located on a narrow peninsula on the Persian Gulf. Its inhabitants converted jointly to Islam in 628, after the ruler of Bahrain and Qatar had become a follower of the Prophet Mohammed. Before that time Zoroastrianism, the ancient religion of the Achaemenids and Sassanids, had been the most common belief, and since the 5th century a few Christians had been living in the region as well. Indian rupees were the most popular currency in the lands around the Persian Gulf and on the Arabian Peninsula in the 19th and 20th centuries. To stop the drain of its money out of the country, India in 1959 introduced the so-called Gulf rupee. Yet when it devaluated the rupee in 1966, the Gulf countries introduced their own currencies. Qatar briefly adopted the Saudi riyal before introducing the Qatar and Dubai riyal together with Dubai. Since 1973 Qatar issues its own independent riyal, however. At the moment there are discussions to replace the Qatari riyal by a single currency of the Gulf states.