§ Erceleb >> Ithil (RS:458,465)
This are earlier names for the substance Mithril.
The first form contains the element er-, on the first glance related to ERE- ’be alone, deprived’ (whence N. ereb ’isolated’, eriol ’alone, single’) and N. celeb ’silver’ (KYELEP-, TELEP-). Since ’isolated’ or ’deprived silver’ does not make too much sense, it might be rather intended to mean *’pure silver’. Compare also the Quenya superlative ĕrĕmelda ’sole dear, dearest of all’ (PE17:57).
In Q(u)enya, at least, the element er was also used in counting, e.g. er ’one (and ’the first’)’ (PE14:49), also Gn. er ’one’ (GL:32) so perhaps Erceleb is *’the first silver’ or even *’the one = the true silver’?.
In the Qenya Lexicon, however, the stem ERE(N) means ’iron or steel’ and the early Qenya names of Angband are Angamandu or Eremandu ’Hells of Iron’ (LT1:249). If Tolkien decided to keep this, Erceleb would be the *’silver-iron’ or *’silver-steel’.
The second form Ithil is plainly ’moon’ (THIL-), but is perhaps rather to be understood as ’silver of the Moon’ (see VT46:18).
Before Mithril was reached there had been another intermediate form Thilevril (see 2.24).