Bitcoins does not earn interest or other capital gain. That just like having dollar bills in your mattress. There is speculative gain, but that is a zero sum game, some looses some wins.
To earn capital gain, you have to either buy capital goods, work factors and produce and sell the result, or do the same through ownership in a company that does the same, or lend out your bitcoins.
It doesn't need to be interest bearing to incur a capital gain. Zero sum is also irrelevant. Options are zero sum and you need to pay gains on options. It also does not have to be a tangible asset, capital gains must be paid on intangible assets (goodwill, patents, etc.).
If I'm not mistaken, anytime the amount realized on the disposition exceeds the purchase price, it's a capital gain.
Edit: To put it another way, if Bitcoin is a currency then like any other Forex trader, gains made would be taxable.