Probably a cleavage piece of kernite. The specimen measures 44x18x5mm.
ncat VHD4 Kernite, Western Borax Mine, Boron, Kern Co., California, USA
Species
Albite (76)
Amethyst (60)
Analcime (117)
Anatase (102)
Anglesite (72)
Aragonite (70)
Azurite (169)
Barite (190)
Brochantite (66)
Calcite (569)
Cassiterite (96)
Cerussite (179)
Chabazite (189)
Chalcopyrite (81)
Chrysocolla (63)
Copper (94)
Crocoite (100)
Cuprite (97)
Dolomite (110)
Fluorapatite (72)
Fluorite (265)
Galena (77)
Goethite (121)
Gold (244)
Gypsum (79)
Hematite (146)
Hemimorphite (67)
Libethenite (86)
Magnetite (61)
Malachite (231)
Mimetite (164)
Natrolite (132)
Phillipsite (138)
Pyrite (238)
Pyromorphite (204)
Quartz (669)
Siderite (129)
Silver (66)
Smithsonite (101)
Smoky Quartz (76)
Sphalerite (119)
Thomsonite (79)
Topaz (58)
Unidentified Species (74)
Wulfenite (153)