I realized after writing yesterday’s post that the reason why Persian reincorporated the Arabic variant (جَوهََر, jawhar) of an originally Persian word (گَوهَر, gawhar) is probably unclear. It’s unclear to me too, not being a linguist or an expert in this kind of thing, but I can offer a guess.
Arabic was the language of the ruling elite in the developing Islamic World from the first Arab conquests of territory outside Arabia, in the 630s, and became the official language of the empire under the Caliph Abd al-Malik, who reigned at the end of the 7th century and died in 705. However, by the end of the 8th century a movement had sprung up called the Shuʿubīyah (شُعُبیه) from the root shaʿb (شَعب, meaning “people,” or “race”) that tried to protect the cultural and linguistic heritage of the peoples who had been conquered by the Arabs. Persian was the focal point of the movement, so much so that, when scholars talk about THE Shuʿubīyah, they are talking about the movement to preserve Persian heritage. This movement dovetailed with the political weakening of the Caliphate in the 9th century, and the rise of Iranian principalities in the eastern (i.e., formerly Iranian/Persian) parts of the empire, principalities that paid nominal allegiance to the caliph in Baghdad, and whose rulers were nominally invested by the caliph, but that were, for all intents and purposes, fully autonomous.
These kingdoms employed an eastern Persian dialect as the spoken language at court, and by the 10th century this “New Persian,” now written in Arabic script, became a literary language through the poetry of Rudaki and, especially, the epic Shahnameh (شاهنامه), composed by Ferdowsi. The new language took hold fairly quickly, and spread even among the Turkic tribes of Central Asia, becoming the lingua franca of the region. In 945, when the Iranian Buyid Dynasty took Baghdad and reduced the caliph to the status of captive pawn, the new Persian got a pretty big boost as it became the official language of the court for political matters. Official letters and documents were written in Persian. Official histories started being written in Persian. Poetry and literature would be written in Persian, starting with the aforementioned Rudaki and Ferdowsi. Arabic had lost its lofty perch as the only literary language in the part of the Islamic world between Iraq and Central/South Asia, and really wouldn’t get it back until modern times, and then only in Iraq (the one majority Arab part of the region I’m talking about).
However, Islam was still a religion whose origins were particularly Arab, and whose holy book was written in Arabic (translations of the Quran have always been considered “interpretations” of the authentic book, OK for study but not of the same holy stature as the original/Arabic Quran). The subsidiary fields of thought that came out of the Quran, primarily theology, jurisprudence, mysticism, and philosophy, were debated and written about by scholars whose work needed to be informed by and read by scholars throughout the Islamic World regardless of any particular region’s official language. Arabic was the natural choice for scholarship in these areas, because any serious scholar of any of them would obviously need to learn Arabic to understand the Quran and all the scholarship that had already been written. This is not to say that there was no Persian scholarship in these areas, but for many centuries a great deal of it derived from Arabic, and many scholars and mystics would write in both languages depending on their audience.
So my guess is that the Persian “gawhar,” meaning “gem” or “essence,” became the Arabic “jawhar,” was used in Arabic mystical/philosophical writing with its more esoteric meaning of “essence,” and found its way back into Persian through those writings.
Reza Shah 
Persian Word a Day 