I had a discussion about timeshare vacations properties, today. Timeshare sales seminars are notoriously high pressure. They're pitched as an investment... but, even though they can be real property, they're a liability.
The litmus test for an investment is simple. If you buy something, today then you can sell it tomorrow for the same price, less a small amount of fees or commission then it's an investment. Real estate (fee simple) and stocks can be bought today and sold tomorrow for virtually the same price. Gold also falls into this category as an investment. However, cars, jewelry (i.e. diamonds, rubies), and timeshares most certainly do not.