Kalmar “rhombohedral” tana nufin siffa ko siffa mai kristal da ke da fuskoki shida, dukkansu suna kamanceceniya, kuma kusurwoyinsa ba duka ba ne. Musamman, ana amfani da shi don kwatanta lu'ulu'u mai nau'i-nau'i guda uku na fuskoki daban-daban waɗanda suke karkata zuwa kusurwoyi daidai da juna, amma ba lallai ba ne a kusurwoyi daidai ba. Ana kuma kiran wannan siffa a wasu lokuta a matsayin trapezohedron trigonal.