This is what is mentioned in the SGI Library:
parable of the jewel in the robe [衣裏珠の譬] (Jpn eriju-no-tatoe )Also, parable of the gem in the robe or parable of the jewel sewn in the poor man's robe. One of the seven parables in the Lotus Sutra. It is related in the "Five Hundred Disciples" (eighth) chapter by five hundred arhats to demonstrate their understanding of the one vehicle teaching. It tells of an impoverished man who goes to visit a close wealthy friend. Being treated to wine, he becomes drunk and falls asleep. The wealthy friend must go out on business, but before leaving, he sews a priceless jewel into the lining of his sleeping friend's robe. When the poor man awakens, he has no idea that he has been given the jewel. He then sets out on a journey. To provide himself with food and clothing, he searches with all his energy, encountering great hardship. Being always in want, he is content with whatever little he can obtain. Later he happens to meet his old friend, who is shocked at his poverty and shows him the jewel in the robe. The man realizes for the first time that he possesses a priceless jewel and is overjoyed. The five hundred arhats explain that, just as this man was ignorant of the treasure he possessed, so the Buddha's disciples were unaware that the Buddha had caused them to plant the seeds of an unsurpassed aspiration and were instead satisfied with provisional teachings and a small portion of nirvana.[quoted from here]
When the sun rises in the eastern sector of the sky, then all the skies over the great continent of in the south will be illuminated because of the vast light that the sun possesses. But the feeble glow of the firefly can never shed light on a whole nation. One who carries in one’s robe a can have any desire fulfilled, but mere shards and stones can confer no treasures. The and other practices, when compared the of the , are like shards and stones compared a precious jewel, or like the flicker of a firefly compared the light of the sun.How can we, whose eyes are dull, ever distinguish the true color of things by the mere glow of a firefly? The fact is that the lesser, provisional sutras of the and True Word schools are not teachings that enable ordinary people attain .And another Gosho, addressed to Toki Jonin:
The persons who had formed ties with the in the days of Buddha forgot the jewel that had been sewn into their robe, and for the period of they wandered in paths of poverty. Those persons who received the seeds of in the even more remote past forgot the good medicine that had been given them, and for the period of they tumbled into the perilous region of the of existence.
The parable is also mentioned in "Sovereign, Teacher and Parent" and "On the five guidelines for Propagation".
Well, that was fun!