A Casascius coin is the name of a certain type of physical – rather than purely digital — bitcoin.

Mike Caldwell, a resident of Sandy, Utah, in the US, first introduced physical bitcoins for purchase in 2011. He coined the name “Casascius” from an acronym for “call a spade a spade.” Caldwell posts updates about his coins on the blog, “Casascius: You asked for change, I gave you coins.”

How does a Casascius work?

Each coin holds the key to the digital value assigned to a particular bitcoin account.

Though the coins are designed to reflect the relative worth of an account, they’re actually just secure containers for the digital information that unlocks the bitcoin’s value. The eight-character code on the outside of the coin corresponds to the first eight characters of an individual bitcoin address, which is assigned specifically to that coin. The individual bitcoin account’s “private key” is embedded on a card inside each coin.

The digital bitcoin, of course, is located online, found in the public “block chain” that records every bitcoin transaction on the internet. The digital coin connected to each Casascius is accessible only to the person who has the private key from the physical coin.

To spend a Casascius, the owner uses the code embedded into it to access the digital bitcoin online. The private key code can be imported directly into bitcoin clients or exchanges like Armory, Blockchain.info or Mt. Gox to deposit funds.

How can anyone tell if a Casascius coin is good?

The coin’s design includes a tamper-evident hologram label that conceals the private key. Peeling off the label to access the key leaves behind a tell-tale honeycomb pattern that indicates the coin has been used.

Caldwell addresses that question on his FAQ page:

The holograms were made by a premier manufacturer who understands they are in the business of making tamper-evident labels, and it looks to me like they did a really good job. It’s pretty difficult to remove the hologram without exposing an obvious “honeycomb” tamper evident pattern. The tamper pattern is extremely sensitive and not concealable once exposed – so sensitive in fact, I ruin over 10% of the holograms just preparing the coins and have to discard them. I don’t know of a way, and I’d be interested to hear if you do.

What does a Casascius look like?

Most look like coins, though there is one denomination that resembles a gold bar:

A ฿0.5 Casascius coin is a brass coin with a one-inch (25.4 mm) diameter.

A ฿1 Casascius coin is a brass coin that’s 1.125 inches (28.6 mm) in diameter (just bigger than a US quarter but smaller than a half-dollar) and weighs a quarter ounce.

1.125 inches (28.6 mm) in diameter (just bigger than a US quarter but smaller than a half-dollar) and weighs a quarter ounce. The ฿25 Casascius coin was minted only in 2011, although it is available for sale. Each coin is about 1.75 inches (44.5 mm) in diameter, around 0.114 inches (2.9 mm) thick and weighs about 1.2 ounces. Combined with the heft, the gold electroplating makes this an impressive-looking coin.

The Casascius 2-Factor Gold-Plated Savings Bar looks like a small gold bar: its dimensions are 8 mm x 40mm x 6mm. If it were made of solid gold rather than a gold-plated metal alloy, it would weigh a substantial 12 ounces … instead of its actual weight of 4.2 ounces. The bar is available as a pre-loaded 100 BTC bar or as a non-denominated savings bar. It includes two-factor encryption.

Where and how can Casascius coins be purchased?

You can buy them online, either with bitcoins or using other forms of payment.

Bitcoins are the only accepted form of payment for Casascius coins on Caldwell’s website. However, other forms of payment are accepted elsewhere, at sites like eBay, BitMit, MemoryDealers, HardBTC.org (UK), Bittiraha.fi (Finland), microbitcoin.fr (France) and BitInnovate.com (Australia).

Is it true that Caldwell stopped taking orders in April 2013?

Yes, but he has since begun taking orders again (he announced this on April 28).

Caldwell has announced some other changes in pricing, batches and delivery options for his 2013 coins:

He has made 8,000 one-BTC token blanks with the year 2013 (and adds he might make more if the coins run out in the middle of the year).

He’s adding a 0.5-BTC coin. “This coin is the same style in every way, but is one inch instead of 1.125 inches in diameter,” Caldwell states, adding, “This is being added due to the rise in bitcoin value. The pricing will be the same as the one-BTC coin (other than being 0.5 BTC less).” The 0.5-BTC coin has a sticker with the same design as the one-BTC coin, only a big smaller.

Caldwell expects 8,000 0.5-BTC token blanks to be delivered soon.

Both the 0.5-BTC and one-BTC coins this year feature a slightly different shape for the private key paper. A lobe has been added to the top edge of the circle to help with orientation.

Caldwell says he has “securely generated 19,800 new addresses intended for these two sets of coins, all starting with a prefix of ‘12’ or ‘13’.” The “13” prefix will designate one-BTC coins, while the “12” prefix will be for 0.5-BTC coins.

How can anyone know the digital keys aren’t copied or used to perpetuate a bitcoin scam?

Caldwell says he puts his own name on the line for that.

He gives his word that he controls the private keys during coin manufacture – inserting them personally or supervising those who do – and asserts that he does not keep any of them. It’s really a matter of accepting him at his word:

I have given out my real-world identity and have digitally signed a list of the bitcoin addresses used in this project. I have made it so that it if I were to perpetrate a scam, it would be possible to prove it and to hold me legally accountable – something no scammer wants to do. You should demand the same from anyone handling your cryptocurrency.

Caldwell has also published a Statement of Controls to answer questions people might have about the integrity of Casascius physical bitcoins. The statement includes details about the coins’ production and funding.