Eleven of the twelve stones on the High Priest's breastplate, as described in Exodus 28, have a reasonably settled modern identification, even where the King James Version's archaic English names, sardius, ligure, and the rest, need translating into terms a mineralogist would recognize. Sardius is carnelian. Jasper is jasper. The ninth stone, assigned to the tribe of Dan and named leshem in the Hebrew text, is the exception, and it has resisted identification for as long as anyone has tried.
The trouble starts with translation itself. The Septuagint's Greek rendering of leshem does not obviously match any of the candidate stones proposed since, and the Latin Vulgate's ligurius, the source of the English "ligure," may reflect a scribal guess as much as a preserved tradition. Later translators and scholars have proposed jacinth, meaning hyacinth zircon; amber, fossilized resin traded into the Levant along ancient routes from the Baltic; tourmaline; and opal. Each has some argument in its favor and none has persuaded the field as a whole.
What makes Ligure a genuine unknown rather than simply an unresolved argument is the total absence of independent evidence that could break the tie. No labeled exemplar from a Second Temple period priestly context has surfaced, and the philological evidence from Semitic cognates has not proven decisive enough to settle between the leading candidates. Until either an archaeological find in secure priestly context or a decisive philological argument appears, the ninth stone of the breastplate remains exactly what it has been for centuries: a name in scripture with no agreed referent behind it.