The tablets are, to a one, made of metal, and of the same metal. It looks like silver, but it's too light and to rigid for it to be that, and still too rigid to be tin, and part of Conrad rebels at the thought that this society is so wealthy that they can use mithril like this, so Conrad isn't really sure what metal it is.
The various tablets that are active are all displaying text describing the features that model of tablet has. There's lots of features available, including something that sounds like it could produce that absurdly detailed picture Maple showed him automatically, but there isn't actually much variation in durability. Fortunately that's because tablets are all made to be nigh indestructible, since they're all still quite valuable.
As for memory, the highest memory he can find is a lot. 'over a million pages' though it doesn't list a specific number and also disclaims that images may occupy more pages of memory than they do visually. That tablet has loads of unnecessary features and costs nearly forty ducats. He can find a very minimal 'writing tablet' tablet with nearly four hundred thousand pages of memory, though, which is also much, much cheaper at only 12 ducats 12 pounds.
Notably, the speaking devices he's seen used a few times are, apparently, a type of tablet as well, and access to the communication network is a feature that many tablets have. You can use the network to transmit images or text as well as sounds, as long as the recipient is equipped to receive them. Tablets that only have voice features are generally the cheapest, and often have very little memory, basically just being a way to access the network.