The work in Nature Communications can hide centimetre-sized objects, limited only by the calcite crystal's size.

BBC: Invisibility cloaking benefits from crystal-clear idea

Iceland spar is a form of calcite that splits light into two beams.

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The stone in question is a piece of Icelandic calcite, and was recovered from a 16th century shipwreck in the English Channel.

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Calcite accomplishes this by sending the two "polarisations" of light - directions in which the light waves oscillate - in different directions.

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This is a large calcite crystal recovered from a vessel that went down off the coast of Alderney, in the Channel Islands, in Elizabethan times.

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It has now been shown that it is of Iceland spar - a form of calcite known for its property of diffracting light into two separate rays.

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And is the calcite reminiscent of daggers or asparagus spears?

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An hour and a half later the group reaches "the dome, " a 60-foot-high semicylinder of rock, one half of the face a deep blue manganese, the other bridal-white calcite.

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But the principle in calcite is different altogether.

BBC: Invisibility cloaking benefits from crystal-clear idea

The team's invisibility cloak exploits this effect, and relies on a specific geometry comprising two prism-shaped calcite crystals glued together, such that they leave a wedge-shaped gap beneath in which objects can be "cloaked" when illuminated with polarised light.

BBC: Invisibility cloaking benefits from crystal-clear idea