BT Šab 90a(22) :ומשתכחא בנוקבא דמרגניתא‏ : it is found in the cavity of a pearl (i.e., inside a mollusk)
See the entire entry for margānī, margānīṯā n.f. pearl

BT Šab 90a(22) :בנוקבא דמרגניתא‏ : the cavity of the pearl [i.e. in which it is located]
See the entire entry for nqe/oḇ, ne/uqbā n.m. perforation