Recent years have seen an emergence of decentralized cryptocurrencies that were initially devised as a payment system, but are increasingly being recognized as investment instruments. The price trajectories of cryptocurrencies have raised questions among economists and policymakers, especially since such markets can have spillover effects on the real economy. We focus on two key properties of cryptocurrencies that may contribute to their pricing. In a controlled lab setting, we test whether pricing is influenced by costly mining, as well as entry barriers to the mining technology. Our mining design resembles the proof-of-work mechanism employed by the vast majority of permissionless cryptocurrencies, such as Bitcoin. In our second condition, half of the traders have access to the mining technology, while the other half can only participate in the market. This is designed to model high concentration in cryptocurrency mining. In the absence of mining, no bubbles or crashes occur. When costly mining is introduced, assets are traded at prices more than 200% higher than fundamental value and the bubble peaks relatively late in the trading periods. When only half of the traders can mine, prices surge much earlier and reach values of almost 400% higher than the fundamental value at the peak of the market. Overall, the proof-of-work mechanism seems to fuel overpricing, which is further intensified by concentration in mining.