People tend to believe that the only thing you can build pentacles on are the metals of the planet. This becomes complicated when you consider that the metals tend to be confused, for example copper is traditionally the metal of Mercury, but due to its affiliation with the island Cyprus where Venus was born became Venus orientated in grimoires. In fact you can make images and pentacles out of pretty much everything. Wax *Art Pauline, parchment *Key of Solomon, clay *Hygromanteia and wood *Key of Knowledge are all mentioned in the grimoires. This particular piece I will list comes from Harleianus 5596 and refers to clay.
"I conjure you, Earth, by your creator, who put you in your place, sealed you with seven seals, and fashioned the man from you; keep your virtue within this clay that I took in my hand, in order to serve me truly and effectively in everything I will make from it. I conjure you, Earth, to obey me truly, sincerely and firmly, as you are firm. Angel Raphael come to my help."
Notes: This is a particularly rich example as it gives further pointers to the order of conjurations to be employed which is confused in later adaptions. Where you see here as I stated was likely in previous posts, that the planet first is to be spoken to and the angel second. You could take Earth to be earth as in mud of course, but as it says "keep your virtue within this clay" you can see that this is actually a planetary prayer. There are very few planetary prayers towards the Earth outside of the extended Juratus, so it is a good find. The manuscript Harleianus 5596 Ioannis believes stems from the 1400's making it one of the oldest forms. Of course the treatise itself clearly dates much older between the 3rd and 8th century.