In “The Song of Flying Fish” by G. K. Chesterton, the author was describing a eccentric but expensive curio, saying:
From the other end of the room it looked like a rather unusually large bowl containing rather unusually large living fish; a closer inspection showed it to be a huge bubble of beautifully blown Venetian glass, very thin and delicately clouded with faintly iridescent colour, in the tinted twilight of which hung grotesque golden fishes with great rubies for eyes. The whole thing was undoubtedly worth a great deal in solid material; how much more would depend upon the waves of lunacy passing over the world of collectors.
What’s meant by “worth a great deal in solid material“, does it mean “a lot of coins“? or “very heavy weight“?
This is a playful way to describe the value of this strange object. The “solid material” in this case is meant to invoke the idea of gold, silver or other precious metal.
It is a sentence about the worth (value) rather than weight.
A related term could be “hard currency” which you should be able to find in a dictionary.