EXCLUSIVE

Pete Davidson shelled out nearly $100,000 for the engagement ring on Ariana Grande's finger ... a piece that was custom made for the singer.

Sources close to the couple tell us Davidson picked up the $93,000 VVS1 clarity 3.03 carat diamond set in platinum last month. We're told it was specially made for Ariana and took nearly two weeks to complete.

The piece was commissioned by NYC jeweler Greg Yuna (aka Mr. Flawless).

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As we reported ... Pete and Ariana got engaged after only dating a short time. Both had previously been involved in long-term relationships -- Ariana with Mac Miller, and Pete with Larry David's daughter, Cazzie.

Miller was busted for DUI last month, shortly after their split went public ... Ariana tweeted hours after the arrest, saying, "Pls take care of yourself."