(c) 2007 Andrew Alden, licensed to About.com (fair use policy)
Lava commonly contains bubbles of gas, which is no surprise because that gas is what causes lava to erupt. The empty bubbles left in the solid rock are called vesicles. Amygdules (a-MIG-dules) are what happens when vesicles later fill with secondary minerals.
Amygdules (often called amygdales by British geologists) fill with a number of different minerals, depending on the groundwater chemistry and the physical conditions underground. These amygdules are full of chalcedony or agate, but the zeolite family of secondary minerals is famous for its occurrences in amygdules. Indeed, they are usually considered to have an amygdaloidal habit.
Notice that some of the amygdules are stretched, not round. These started out as spherical vesicles and were deformed while the lava was still fluid. Amygdules and vesicles thus can be indicators of movement in ancient lavas.
A rock with amygdules is said to have amygdaloidal texture. The word comes from the Latin for "almonds" and refers to their typical shape.