"They are getting more serious," says source.

Amber Heard and Elon Musk Step Out in L.A.: 'It's Definitely Not a Casual Relationship,' Says Source

After spending some family time with Elon Musk in Australia, Amber Heard and the tech mogul were spotted together back in Los Angeles over the weekend.

The couple, who had spent time together Down Under last month while Heard, 31, filmed Aquaman, had lunch at Sweet Butter in Los Angeles on Friday. “They were very casual and dined in,” an onlooker tells PEOPLE. “Elon had his arm around Amber as they waited in line.”

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The eyewitness adds, “They ordered at the counter and sat down outside to wait for their salads. Elon paid and was pretty chatty. It was his first time at the restaurant so he asked questions about the menu. They were both very friendly and seemed happy.”

Dating rumors began swirling last year when Heard and Musk, 45, were spotted out together several times amid their respective divorces — Heard’s contentious divorce from Johnny Depp was finalized in January, and Musk filed for divorce from his second wife, Westworld actress Talulah Riley, in January 2015.

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“They are getting more serious,” says a source close to the couple. “It’s definitely not a casual relationship. That’s not Elon. He doesn’t do anything casually.”

The source confirms that the Tesla founder “isn’t dating anyone else” and “is still very fascinated by Amber and loves her company. He seems very happy with her. She is spending a lot of time with his kids.”

While working in Australia, Heard spent a day sightseeing with Musk’s five sons. The actress shared a few snaps from the day out, showing the group goofing off together at the Sydney Opera House. Musk’s sons — Damian, Kai, Saxon, Xavier, Griffin — are from his marriage to his first wife, Justine. (The couple had another son, Nevada, who died at 10 weeks.)