Would it be possible to manifest a weapon with the Silvered or Adamantine property?
According to the description of Pact of the blade:
You can use your action to create a Covenant weapon in your empty hand. You can choose the form this melee weapon takes each time you create it (see Chapter 5 for weapon options).
Chapter 5 of the Player's Manual is about equipment; including weapons. In weapons, there is a subsection on silver weapons.
Some monsters immune to or resistant to non-magical weapons are susceptible to silver weapons. Therefore, cautious adventurers invest an extra coin to put on their silver weapons. You can use a single weapon or ten pieces of silver for 100 inches. This cost represents not only the price of money, but also the time and expertise needed to add money to the weapon without making it less effective.
If it was a physical thing, then it would have to spend money for the process of money. But it is a weapon created from the spirit of the wearer and formed from Shadow.
Note: I understand that the money a weapon is used to defeat the resistance and that the manifest weapon is considered magical, which usually helps to overcome the same resistance. But I could imagine a monster really vulnerable to money, so money would be an advantage beyond magic.
In addition, if a weapon has adamantine ownership, it is better at breaking objects. Another bonus above is just magic.