Musk Is Diversifying World’s Biggest Fortune With Twitter Stake (1)

For years, Elon Musk’s net worth has been dominated by one stock: Tesla Inc.

His purchase of a 9.2% stake in Twitter Inc., disclosed Monday in a regulatory filing, adds an unusual element of diversification to his $288.4 billion fortune.

Musk’s holding in his favored social-media platform is worth $3.67 billion. He’s already up more than $1.2 billion on the position based on Twitter’s closing share price on March 14, the date of the event that triggered the disclosure, and its closing level on Monday, when the stock surged 27% in the biggest rally since the company’s 2013 trading debut.

Even though Musk ...