Ulexite
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Ulexite sometimes called TV rock or TV stone due to its unusual optical properties, is a hydrous borate hydroxide of sodium and calcium with the chemical formula NaCaB₅O₆(OH)₆·5H₂O. The mineral occurs as silky white rounded crystalline masses or... Wikipedia
2V angle: Measured: 73–78°
Birefringence: δ = 0.028
Category: Nesoborates
Cleavage: Perfect on, good on, poor on
Fracture: Uneven
Luster: Vitreous; silky or satiny in fibrous aggregates
Optical properties: Biaxial (+)