The Collaborative International Dictionary of English v.0.48:

Amygdaloid \A*myg"da*loid\, n. [Gr. ? almond + -oid: cf. F.
   amygdalo["i]de.] (Min.)
   A variety of trap or basaltic rock, containing small
   cavities, occupied, wholly or in part, by nodules or geodes
   of different minerals, esp. agates, quartz, calcite, and the
   zeolites. When the imbedded minerals are detached or removed
   by decomposition, it is porous, like lava.
   [1913 Webster] Amygdaloid

The Collaborative International Dictionary of English v.0.48:

Amygdaloid \A*myg"da*loid\, Amygdaloidal \A*myg`da*loid"al\, a.
   1. Almond-shaped.
      [1913 Webster]

   2. Pertaining to, or having the nature of, the rock
      amygdaloid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

amygdaloid
    adj 1: shaped like an almond [syn: almond-shaped,
           amygdaliform, amygdaloid, amygdaloidal]
    n 1: volcanic rock in which rounded cavities formed by expanding
         gas have subsequently become filled with mineral deposits