Jewel of the Commonwealth

Jewel of the Commonwealth is a main story quest in Fallout 4.

Detailed walkthrough
This quest becomes available at the conclusion of When Freedom Calls quest. Alternatively, the player may simply happen across Diamond City while exploring. Either way, the Sole Survivor will be told that Diamond City is the best hope for gaining further information about Shaun. Getting there from Sanctuary Hills is a long trek southeast. Upon crossing the bridge into the remains of the city, look for signs pointing in the direction of Diamond City.

At the entrance, the Sole Survivor will witness a confrontation between Piper Wright, the owner of the city's local newspaper Publick Occurrences, and McDonough, the mayor of the city. Piper will use the Sole Survivor to trick the guard into open the gate, thus letting them both in.

Speaking further with McDonough, a moderate-difficulty Speech check will get him to tell the Sole Survivor who to talk to about Shaun, then head into Diamond City market.

Asking any of the following NPCs provides a clue to the Survivor's next move:
 * Danny Sullivan - the gate guard.
 * Pastor Clements - of the All Faiths Chapel.
 * Nat Wright - Piper's kid sister, selling copies of Publick Occurrences.
 * Piper Wright - after being interviewed.
 * Moe Cronin - Swatters salesman extraordinaire.
 * John - the barber.
 * Arturo Rodriguez - the weapons merchant.

Speaking with any of those people will direct the player character to the Valentine Detective Agency. However, upon arrival at Valentine's place of business, his secretary, Ellie Perkins will inform the Sole Survivor that Valentine disappeared while working on a case. Ellie hopes the Sole Survivor might lend a hand to help find him. Charming Miss Perkins will allow the player character to shake her down for a few caps upon delivery of her boss. In any case, agreeing to help will finish the quest and start Unlikely Valentine.