Martin Lawrence Drops $500k On Engagement Ring

Martin Lawrence didn't cut corners on his engagement to Roberta Moradfar -- instead he plunked down about a half mil for her rock .... TMZ has learned.

Sources connected to the couple tell us Martin worked for months with Iconic Jewels Beverly Hills to create the custom 5 carat emerald cut engagement ring. He added another carat for the band alone, which has Martin and Roberta's initials spelled out in diamonds.

Martin popped the question Friday night to Roberta, a nurse practitioner ... just a few hours after the ring was delivered to his home.

This will be his third marriage, but he's clearly not gun-shy.

Mazel.