฿1 Casascius Coin: This is a solid brass coin. Each 1-bitcoin coin is about 1.125inch (28.6mm) in diameter (just bigger than a US quarter but smaller than a half-dollar) and weighs a quarter ounce. Perfect as a small gift to introduce someone to Bitcoin. Also available in a ฿0.5 version which is slightly smaller at 1 inch (25.4mm).
฿1 Gold-Plated Fine Silver Casascius Round. This is a 39mm 1oz silver round accented with gold electroplating on the rim and on the Bitcoin logo, loaded with one digital bitcoin.
฿0.5 and ฿0.1 Fine Silver Casascius Rounds These are a half-ounce and quarter-ounce rounds (respectively) of fine silver. Diameters are 30mm and 25mm.
Casascius 2-Factor Gold-Plated Savings Bar: Dress your Bitcoins for tomorrow, make them look their best in your vault today. Would weigh about 12 ounces if it were solid gold, this is a 4.2-ounce metal alloy bar with gold plating. A neat-looking novelty that looks unmistakably valuable. Available as a pre-loaded 100 BTC bar, as well as a non-denominated savings bar. Two-factor encryption is available at no charge. Bar is 80mm x 40mm x 6mm.
How they work:
The "private key" is on a card embedded inside the coin and is protected by a tamper-evident hologram. The hologram leaves behind a honeycomb pattern if it is peeled. If the hologram is intact, the bitcoin is good. If you have purchased a 2-factor item, the private key is encrypted and will need to be decrypted using your original preselected passphrase before you can redeem the funds.
The 8-character code you see on the outside of the coin is the first eight characters of the Bitcoin address assigned specifically to that coin. You can verify the coin's balance on Block Explorer. There is a mathematical relationship between the Bitcoin address and the private key inside the coin. The digital bitcoin is actually located on the public "block chain" stored on the internet, but it is completely inaccessible to anyone unless the private key from the coin is loaded into a Bitcoin wallet.
To recover the digital bitcoins, there are several ways to convert the embedded code back into a digital bitcoin so it can be spent over the internet. Most importantly, none of the methods relies on me or any other central issuer, due to Bitcoin's completely decentralized design. The embedded private key code is everything a Bitcoin client needs to find and claim the digital Bitcoins from the peer-to-peer network. For example, you can enter (or "import") your coin's private key code directly into Bitcoin clients such as Armory, Blockchain.info, or directly into Mt. Gox as a deposit method. (Casascius coins use the "minikey" private key format, and the main Bitcoin.org client does not yet support redeeming minikeys.)
Of course, since the face value of the coins depends on the integrity of the embedded key code, you should only accept Casascius Bitcoins bearing an undamaged Casascius hologram from others.