You are putting the emphasis on 'speculative' rather then 'bubble'. All speculative markets go through bubble-like phases, and your success in speculating in a market depends quite a bit on your ability to understand what is driving prices in that particular market during the times you are active in them, and in understanding when, and to what extent, they are bubbling. If one thinks tulips are just like art is just like bitcoin is just like cattle futures, you may not be well-enough informed to speculate successfully in any of those markets.