RIYADH, Jan 23 (Reuters) - Thailand"s prime minister will visit Saudi Arabia on Tuesday, the Saudi foreign ministry said, in what will be the first high-level meeting between the two countries since a diplomatic row over a jewellery theft nearly three decades ago. Saudi Arabia downgraded its diplomatic relations with Bangkok following the theft in 1989 of around $20 million of jewels by a Thai janitor working in the palace of a Saudi prince, in what became known as the “Blue Diamond Affair”. A large number of the gems, including the rare blue diamond, are yet to be recovered. Thailand"s Premier Prayuth Chan-ocha will start a two-day visit to Saudi Arabia on Tuesday at the invitation of Saudi Crown Prince Mohammed bin Salman, the Saudi ministry said in a statement on Sunday. "The visit comes amid consultations that led to bringing views closer on issues of common interest," the ministry said. The visit is aimed at coordinating on those issues, it said, without elaborating. The theft of the jewels remains one of Thailand’s biggest unsolved mysteries and was followed by a bloody trail of destruction that saw some of Thailand’s top police generals implicated. A year after the theft, three Saudi diplomats in Thailand were killed in three separate assassinations in a single night. A month later, a Saudi businessman, Mohammad al-Ruwaili, who witnessed one of the shootings, disappeared and later in 2014, a Thai criminal court dismissed a case against five men, including a senior police officer, charged with murdering Ruwaili over the precious stones. Thailand has been eager to normalise ties with the oil-rich Kingdom after the spat that has cost billions of dollars in two-way trade and tourism revenues and the loss of jobs to tens of thousands of Thai migrant workers.