Pyramid schemes are defined by the price and structure. A business that sells knives is a fine business. A business that sells overpriced knives by promising that you can then find someone else to sell more knives for you at an even higher price is a pyramid scheme.

Selling tulips is a fine business. Selling tulips at an insanely high price by promising that the market for tulips will keep on expanding and increasing the price of tulips is a pyramid scheme. (Well, maybe not quite a pyramid scheme, the structure isn't right. But it certainly wasn't a sustainable business model.)