As nouns the difference between amygdaloid and mandelstein

is that amygdaloid is a variety of trap or basaltic rock, containing small cavities, occupied, wholly or in part, by nodules or geodes of different minerals, especially agates, quartz, calcite, and the zeolites when the imbedded minerals are detached or removed by decomposition, it is porous, like lava while mandelstein is amygdaloid.

As an adjective amygdaloid

is shaped like an almond.