Ancients people would sometimes chisel (test cut) a coin to determine if the coin was a good, official, solid silver or gold coin, or if it was a fourree, a plated counterfeit.
Unfortunately, the damage caused by test cuts detracts from the eye appeal, and thus the value of cut coins to modern collectors. There is no formula to determine the impact on value, but the reduction in value corresponds to the loss of eye appeal caused by the cut. The location and size of the cut clearly matter. A tiny edge cut may have very little impact on value. A cut that nearly splits a coin in two or that obliterates details of the type will likely slash (pun intended) the value of the coin. The impact on value of a test cut also depends on grade and preservation of the coin. A test cut will not have much effect on the value of a slug (a very worn coin), which had little eye appeal anyway. On the other hand, a coin that would otherwise be choice extra fine, with excellent eye appeal, will likely be dramatically reduced by a test cut. From a positive perspective, test cuts make some specimens affordable to collectors who might not otherwise be able to afford the type.
Test cuts may be found on nearly all types of ancient precious metal ancient coins, but they are most common on Athens tetradrachms. Some "Athens" tetradrachms were actually struck in Asia Minor, and Egypt. These tetradrachms are imitative, not official Athens issues. Most of these imitatives are good silver and are so close to Athens mint style that they are listed as ordinary Athens tetradrachms in auctions and sales catalogs. Some of these apparent imitatives are, however, actually plated counterfeits.
"To deal with [counterfeits], the Athenians passed a law in 375/4 B.C. which provided for a dokimastes or 'tester' to sit near the banking tables in the Agora and in the market of Peiraieus. The judgment of this official as to the authenticity of any disputed piece was final. Any owl which was of silver and correct weight, whether it was struck in Athens or at a foreign mint, had to be accepted in commerce. Counterfeit pieces, on the other hand, were slashed by the dokimastes, withdrawn from circulation and dedicated to the Mother of the Gods. Such counterfeit owls have, in fact, been found near the Metroon, sanctuary of the Mother of the Gods." -- Greek and Roman Coins in the Athenian Agora, American School of Classical Studies at Athens (Princeton, NJ), 1975. The dokimastes was apparently was able to verify most coins without cutting and most of the cut coins found at Athens are plated.
Most test cuts were probably not made by government employees, they were made by merchants and money changers. Test cuts are very common on classical "old style" Athenian tetradrachms and other classical coins found in Egypt. In the classical period, Egyptians did not appreciate coinage and viewed coins essentially as bullion. They would routinely cut coins both to test them and to cut them into pieces to make change.
Because test cuts impact value, some test cut coins, such as on the coin below, that have been "restored" or "repaired." The cut was originally through the owl's head, which is most common location for Athens tetradrachm test cuts. Dealers and experienced collectors view test cut repairs, not as restoration, but rather as additional damage. Such coins are completely unacceptable to most collectors.
The "give-away" that makes some test cut repairs very obvious is a flat area on the other side of the coin opposite where the original cut was made. Examples of these flat areas are visible, circled in yellow, in the photos below.