Is a bitcoin worth the $15,000 it commands today, or is it really worth about $3,000? The huge runup in value since September suggests the lower figure.

Cryptocurrency fans typically fall into two groups. One sees the currencies as ways to buy and sell things; the other views them as investments. For now, the investment crowd is winning out: Bitcoin remains a cumbersome way to purchase most goods, but its value has skyrocketed, nearly quadrupling since mid-September.

If bitcoin is an investment, it most closely resembles gold. Both are stores of value that provide some built-in protection against inflation because there is a finite supply and because extracting new deposits gets more expensive over time, barring big technology changes.

If bitcoin really is digital gold, however, investors should analyze it like a commodity—looking at supply constraints and the factors driving demand. That exercise produces worrying results.

The most important factor in gold prices over the long run is production costs, which act something like a natural price floor when demand dips. Of course, gold prices can also temporarily move much higher when demand is strong but tend to fall back toward the marginal cost of production once worries about inflation or the dollar subside and gold begins to lose its appeal as a hedge.