Describing the city
Strabo described the city as having "fancy tools made out of gold and silver, such as the family gold, right [Qawa'im] triangles, and their drinking glass, let alone their large homes which have their doors, walls, roofs filled with colors, gold, silver, and holy stones" 
Location and Etymology
To the Ancient Greeks, East Arabia (current-day al-Hasa province) or its capital city was known as Gerrha, but Gerrha was a Greek alteration of Arabic Hagar (current day Hofuf) the name of the largest city of Ancient Bahrayn (Bahrayn was also known as Hagar or Gerrha in Hellenistic times). other English spellings are "hajar hofuf, hagar hasa' hagara". Hagar (Gerrha) is not to be confused with West Arabia Al-Hijr (al-Hegra, Hegra ) (current day Madaen Saleh or al-Ula near the Red Sea.
Hagar (Gerrha) was destroyed by the Qarmatians at the end of the ninth century where all 300,000 inhabitants were massacred. It was 2 miles from the Persian Gulf near current day Hofuf. The researcher Abdulkhaliq Al Janbi argued in his book that Gerrha was most likely the ancient city of Hajar, located in modern-day Al Ahsa, Saudi Arabia. Al Janbi's theory is the most widely accepted one by modern scholars, although there are some difficulties with this argument given that Al Ahsa is 60 km inland and thus less likely to be the starting point for a trader's route, making the location within the archipelago of islands comprising the modern Kingdom of Bahrain, particularly the main island of Bahrain itself, another possibility.
Origins of the inhabitants of Gerrha
As Gerrha is located in the Arabian Peninsula, there's no doubt that the city's inhabitants were Arab. All scholars, except Strabo, who described the inhabitants as "Chaldean exiles from Babylon", he later retracted his statements when he said "Because of their trade, the Gerrhans became the richest of the Arabs", agree that the inhabitants were indeed Arab. Also, petroglyphs were found in Greece and were found out to be sent by a man from Gerrha called Taym Al Lat, which is undoubtedly an Arab name.
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