made of or from opal (Hydrated silica. SiO2Â·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 jarÄkerel.
pa jakÄ«Ã¾a anrÄkerel;
The rock contains opal.
More literally, the rock has an attribute: opal.
la jakÄ«Ã¾a jarÄkerel;
The rock is opal. (or, The opal rock is.)
A picture (by me, taken in Australia):