Three of the four ingredients Exodus 30 names for the incense of the sanctuary are reasonably secure. The fourth, shecheleth, is not. The Septuagint renders it onycha, which means fingernail or claw, and every identification since has been an argument about what that word was translating.
The leading candidate is the operculum, the horny plate with which certain marine gastropods seal their shells. It does look like a fingernail, it was traded as an aromatic in the ancient Near East under names that fit, and treated with an alkali and burned it gives a musky animal smell that works in a compound. Arab and later European sources describe exactly this material under the name blatta byzantina.
The difficulty is halakhic. A sea snail is not kosher, and an ingredient of the temple incense being a non-kosher animal product needs explaining. Several authorities accept it on the ground that the incense is not eaten; others reject the identification for that reason and look elsewhere.
The alternatives are all plants: benzoin, labdanum, a rockrose resin, or the bud of a clove-like tree. None of them accounts for the fingernail sense of the Greek.
The stakes are practical. Any attempt to reconstruct the ketoret has to decide, and there is no evidence that would settle it short of a residue analysis from a temple-period vessel.