01Decisive
The doubled line
Lay a clear rhomb over a single sharp line and look down through it. Two distinct lines appear, and rotating the crystal moves one around the other. No common transparent mineral does this as strongly.
Calcite · Lovenhall Stone
How to Identify Iceland Spar
Optically clear calcite. The classic double-refraction specimen, and a real object in the history of optics.
Everywhere. It builds limestone, marble, chalk, stalactites and most shells
Iceland spar is transparent, colourless calcite, historically from Helgustaðir in eastern Iceland, and it is the material through which double refraction was first properly studied. Erasmus Bartholin described the effect in it in 1669, and it later served in Nicol prisms and polarising instruments before synthetic materials replaced it.
Its identification is its whole point. A clear rhomb laid over a printed line splits the image into two, and the separation is wide enough to be unmistakable at a glance rather than needing instruments. Rotate the crystal and one image orbits the other.
It is also a good demonstration piece for the other two calcite tests, since it is soft enough to mark with a coin and will fizz on a corner. Worth noting that the classic Icelandic locality is long exhausted and modern material comes mostly from Mexico and China, sold under the same name.
Model-specific checks
What decides a Iceland Spar
These apply to this model and not to the rest of Calcite. Work them alongside the maker's general checks, not instead of them.
The market
What a Iceland Spar does on the secondary market
Inexpensive. Good clear rhombs run $5 – 40 by size and clarity.
An orientation for honest, unrestored condition — never an appraisal, and never a quotation.
Questions we are asked
- Why does calcite double an image?
- Because it is strongly birefringent: light entering the crystal splits into two rays travelling at different speeds and slightly different paths, so each point of an image is reproduced twice. Calcite's birefringence is unusually large, which makes the effect visible to the naked eye rather than requiring instruments.
All the Calcite checks
Free tools of the room
Lovenhall StoneRead the stone itself
The tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.
Free on iPhone & iPad. An estimate, never an appraisal.
Or write to the house