Side projects are not just 20-somethings coding away in their dorm rooms or the young father who makes his wife angry spending all day at work and every night on his computer. Seth Rosen’s side-project, the International Gem Society (IGS), fits neither stereotype.

Rosen is the co-founder of CustomMade, an online marketplace backed by Google Ventures’ that connects customers with independent artisans producing custom-designed furniture, jewelry, home décor and other personalized items. Rosen is a die-hard collector of fine gemstones and a well-known private dealer.

Related: 5 Tips for Launching a Side Business

Rosen spends a few hours weekly on the International Gem Society website. The site receives more than 64,000 unique page views per month, according to its Google Analytics dashboard. Rosen works on the website with his wife, Lisa, who manages the operations, programming, membership and certification programs while he focuses on content strategy and growth.

More than half of the site’s 64,000 unique page views are from organic search. There are no referrals from Facebook or Twitter because up until last week Gem Society was not on Facebook and is still not on Twitter. Rosen sees his side-project as a hobby and an investment. 

Related: Entrepreneurs Who Turned Hobbies into Million-Dollar Businesses

Entrepreneurs have always had their side projects. Here is Rosen’s advice for entrepreneurs and their side projects.

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One Entrepreneur's Gem of a Hobby (Infographic)