made of or from opal (Hydrated silica. SiO2·nH2O).
Names of substances, including minerals, are generally given in the stative, as they are considered to be attributes. A piece of opal would be referred to with the singular jarākerel.
pa jakīþa anrākerel;
The rock contains opal.
More literally, the rock has an attribute: opal.
la jakīþa jarākerel;
The rock is opal. (or, The opal rock is.)
A picture (by me, taken in Australia):