The root meaning of s-b-k has always been associated with metal processing. E.W. Lane An Arabic–English Lexicon, 8 parts, (London 1872), IV, p. 1300, s-b-k ‘to melt’; sabīka, pl. sabāʾik ‘ingot, piece of gold or silver of an oblong form that has been melted, cleared of its dross and pour forth into a mould (misbaka) of iron, like the half of a cane divided lengthwise.’
On the perception of an ingot’s smooth form, Lane adds that ‘an Arab of the desert likened to it a difficult mountain that he desired to ascend because of its smoothness, saying “what an ingot this is!”.’ Cf also Kazimirsky, I.1047 sabīka, sabāyik ‘lingot’. The same meaning is found in medieval Arabic of al-Andalus; see F. Corriente, A Dictionary of Andalusi Arabic (Leiden 1997), p. 243: s-b-k ‘to found, cast, smelt’; sabīka, sabāʾik ‘bar, ingot’.
Modern Arabic vocabulary echoes the medieval in which sabka refers to the casting of ingots or bullion whereas native silver or gold is referred to as ‘a cutting’ or ‘piece’; hence, qiṭʿa al-fiḍḍa for ‘silver nugget’.