Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.047.

The term rikaz is etymologically derived from rakaza, the perfect tense of the verb yarkuzu (the imperfect root). It means ‘to be hidden.’ Allah, the Exalted One, says: “Or hear from them the slightest sound” [Maryam 98] – that is, rikz means a slight sound.

In the present context, this refers to what was buried at the time of jahiliyyah (the pre-Islamic period). Malik and many other scholars are of the opinion that rikaz means objects buried before the Arabs embraced Islam and which were dug up without any expensive effort or money. If these conditions cannot be met, then it is not considered rikaz. Abu Hanifah holds that it is a name of an entity hidden either by the Creator or by the created one (man).