Bitcoin is digital, yet this image showing physical bitcoins ranks as the most popular photograph of the virtual currency on Gettyimages.com.

Dollars, euros and pounds are easy to visualize: Theyre physical objects. Being digital, bitcoins arent tangible -- which has become a boon for George Frey.

Ive sort of cornered the market on bitcoin photos, the Provo, Utahbased photographer says in the November issue of Bloomberg Markets magazine.

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Indeed, dozens of media outlets, including Bloomberg News, have published Freys pictures. The image used here -- dated April 26, 2013 -- happens to rank as the most popular bitcoin photograph on Gettyimages.com. But if a bitcoin isnt a physical object, what exactly are we looking at in Freys photographs?

Theyre called Casascius bitcoins, and they were minted, in a variety of metals, by software engineer Mike Caldwell at his home in Sandy, Utah. An early bitcoin adopter, Caldwell wanted to help popularize the cryptocurrency -- but he, too, grappled with its intangibility.

No one is going to get this if I cant show them something, Caldwell remembers thinking.

So in September 2011, he began making physical coins as vessels. Inside of each, he embedded a piece of paper that contained a bitcoin private key, which he protected with a tamper-resistant hologram sticker. As for the word casascius, it was half acronym (derived from the phrase call a spade a spade), half Latin-sounding suffix (cius).

As of Nov. 27, 2013, however, Caldwell no longer includes digital bitcoins in his physical bitcoins. A letter from the U.S. Treasury Departments Financial Crimes Enforcement Network, or FinCEN, stopped him in his tracks: FinCEN considered his activity to be money transmitting and informed him that he lacked the necessary license.

Since then, Caldwell has sold only aluminum promo coins via his website, Casascius.com. A bag of 500 costs 0.39 bitcoin -- about $150, as of yesterday. All told, Caldwell minted about 60,000 Casascius bitcoins. Bitcoin enthusiasts consider them collectibles -- especially the earliest ones, which have fetched as much as $2,500 each on EBay because of a typo in the hologram.

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Whats a Bitcoin Look Like? Popular Photograph Has Story