The Rothchilds were/are arguably the most successful (in a financial sense) family in history. They've been able to carry on that phenomenon for well over 150 years.
In Niall Ferguson's biography on the family he iterates an investment maxim by the family. You can see it below:
1. One third of your wealth should be in securities
2. One third of your wealth should be in real estate
3. One third of your wealth should be in jewels and artwork
This was well over 150 years ago and I need to modernize it for everyone. The general principle of diversification is the same and that will never go away. That's why it's a principle.
I like 1 and 2. However the problem that I have is with 3. Jewels and art (like gold and other precious metals) produce no cash flow. So it would be good to substitute business ownership for 3.
Now this could be a business that you own and operate or it could be one that you just have an ownership stake in. Regardless it should be generating cash flow. That's what you have to be looking for with any investment.
On top of that it can't be at the expensive of depreciating assets. Here's an example. You buy an asset that has a useful like of 5 years after which it is worth nothing. It cost you $50,000. In those 5 years you generate $40,000 in free cash flow. Bad investment! That's not generating cash flow even though it appears to be.