The “root” ᴹ√TINKŌ (more likely just a primitive word) appeared in The Etymologies of the 1930s with the gloss “metal” and derivatives like ᴹQ. tinko/N. tinc of the same meaning (Ety/TINKŌ). The reappearance of Q. tinco “metal” in The Lord of the Rings appendices (LotR/1122) strongly indicates its ongoing validity.
ᴹ√TINKŌ may have replaced the root ᴱ√SINI “pale blue” from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. sink “mineral, gem, metal” and G. sinc “metal” (QL/83; GL/67); these early mineral words were originally attributed to ᴱ√SṆT͡YṆ “twinkle” before they were transferred to ᴱ√SINI, leaving only derivatives like ᴱQ. sintl “crystal” and ᴱQ. sinty- “sparkle” under ᴱ√SṆT͡YṆ (QL/85). However, in notes from the late 1960s Tolkien had primitive ✶sinki as an element ✶sinkitamo, the basis for Q. sintamo “smith” (PE17/108). Likewise there is evidence of the earlier root in Q. sinca “flint” as in Q. sincahonda “flint-hearted” (LotR/979), initially given as ᴹQ. tingahondo in Lord of the Rings drafts (SD/68).
Neo-Eldarin: For purposes of Neo-Eldarin, I would definitely use ✶tink- = “metal”, but I think it is worth keeping ✶sinki = “✱mineral = any inorganic solid including both stone and metal” as a variant.
An onomatopoeic root with variants ᴹ√TING and ᴹ√TANG appearing in The Etymologies of the 1930s with derivatives ᴹQ. tinge or tango “twang”, ᴹQ. tinga- “to twang”, and N. tang “bowstring” (Ety/TING). The entry had a reference to a deleted variant ᴹ√THING (EtyAC/TING) and another variant ᴹ√TUNG was listed in the entry ᴹ√DING (EtyAC/DING). In the Quenya Verbal System (QVS) from 1948 Tolkien gave the root ᴹ√TAÑ as the basis for the verb ᴹQ. tanga- “twang”, which is probably another instance of this root (PE22/103).