Casascius Physical Bitcoins or Casascius coins are physical metal coins created by Bitcoin user Casascius (Mike Caldwell, Sandy, Utah, USA) and solduntil Nov 26, 2013. It contains an embedded piece of paper with digital Bitcoin value, covered by a tamper-resistant hologram. Casascius coins are available inthe denomination of 1, 10, 25, 100, and 1000 BTC increments.

Each Coin has its own Bitcoin address and a redeemable "private key" which is covered by a hologram. To read the private key inside the coin the person needto peel the hologram, once peeled and redeemed the coin will no longer have any Bitcoin value. These coins are designed for face-to-face transactions.

The initial success of the 1 BTC Coins inspired Mike to create physical bitcoins of other denominations

Suspension of sale
The Financial Crimes Enforcement Network (FinCEN), a branch of the Treasury Department, informed Casascius that minting physical coin and transmitting moneyrequires state license and federal level registration. And thus from Nov 27, 2013, onwards, the sale is suspended for items that contain digital bitcoins, thecurrent items in the sale does not contain bitcoins.

Fake Casascius Coins
There have been multiple reports of fake Casascius coins being circulated in the market. The counterfeit and unauthorized coins even appear to mimic thespelling error from the Series 1 coins. Check out this bitcointalk link where the author compares an original Casascius coin with a fake one.

One should not accept Casascius Bitcoins if the hologram is damaged because the integrity of the coin depends upon the hidden embedded key code inside thehologram.

To read the complete article, see:
What isthe Casascius Physical Bitcoin? Exploring an important part of Bitcoin's history(https://www.altcoinbuzz.io/bitcoin-and-crypto-guide/what-is-the-casascius-physical-bitcoin-exploring-an-important-part-of-bitcoins-history/)

To read the earlier E-Sylum articles, see:
BITCOINS GO FROM VIRTUAL TO PHYSICAL (https://www.coinbooks.org/esylum_v14n45a22.html)
U.S. GOVERNMENT SHUTS DOWN PHYSICAL BITCOIN MINTER(https://www.coinbooks.org/esylum_v16n51a14.html)