Edit: My original question concerned the magnetic properties of gems in this world. As I have realized more and more through the answers how unrealistic this is, I am now seeking input on any general phenomena that could achieve my desired outcome.
An archipelago located south of Australia is rediscovered by a modern team of explorers. This island was for all of recorded history, hidden in plain sight due to a phenomena that concealed it from view. It was accidentally discovered by Lemurian explorers in antiquity, but there was never any colonization that followed. The island has unique geological minerals that may or may not have something to do with its unique properties that keep it hidden.
As it stands, I want to make this more realistic. I just need help thinking of ways how to! What phenomena could hide an archipelago the size of Tasmania?
The Alkumateki Islands formed 50 million years ago from leftover continental fragments after Australia separated from Antarctica during the Cretaceous period. Geologically, the islands have strong geological compositions from the Gondwana continent which once linked Australia and Antarctica together.
Today, the islands consist of three main islands of roughly similar size, a miniscule amount of islets, and a handful of seamounts. The three main islands are located at a more southern latitude than Tasmania, but apart from the third do not reach any further towards Antarctica. The third is naturally the coldest of the three, but their collective proximities to the icy continent have a dramatic impact on all their respective climates.
Two of the islands are around the same size as Tasmania, while the third is comparatively small and only 2/10ths the size. Together, they are similar to the size of Honduras.
Underneath the islands are vast caverns of unique endemic minerals which were exploited by the Lemurians following their exile from their lost continent. When used properly, they had unique magnetic properties which helped to propel the Lemurians into a golden age before their society collapsed in a civil war circa 917 BC.
My fundamental question is this: How could the island or its geological materials help conceal it from compasses, radars, and satellite imagery?