From: 10th Ore Zone, Kerr McGee Mine, Carlsbad, New Mexico, USA
Size: 6.4 x 3.5 x 3.2cm
Good blue halite specimens from Carlsbad are getting harder and harder to find since there hasn't been any mining there in years, so the supply has really become scarce. There are basically two types of blue halite specimens that you'll see for sale: slabs and crystal cubes. The slabs are fairly inexpensive and much easier to find but still beautiful, but the crystal cubes like this specimen are what are most valued by collectors.
The aesthetics of this specimen are great! A large, clear sylvite crystal measuring 4.8cm wide has a blue haite cube attached to its side. The blue halite crystal is 2.5cm on it's long edge. The condition is excellent, and the blue halite crystal has a nice deep blue color and interior transparency. The sylvite is clearer in person than in the photos because the studio lights cause a lot of refraction in that big crystal.
The blue halite is chemically the same as the clear halite - the color is not caused by any additional chemical elements in the salt. The blue is actually a result of tens of millions of years of exposure to the radioactive potassium in the sylvite (Potassium Chloride). This is the same potassium you'd find in a banana, and it has a half-life of a billion years, but over tens of millions of years of exposure it builds up and the atomic modifications to the crystal structure are what cause it to appear blue. If you were to dissolve it and let it re-crystalize it would look white just like regular salt.