glass bead; glass marble; glass sphere
Refers to small glass balls used as marbles, beads, or decorative spheres. The context usually makes the specific type clear.
When I was a child, I collected glass marbles.
Blue glass beads are used in that necklace.
ビー玉 specifically means a small glass marble used in children's games, while ガラス玉 is the broader term for any glass bead or sphere.
Most common spelling using katakana for ガラス and kanji for 玉.
Alternate spelling using 球 (sphere/ball).
Rare spelling using the kanji 硝子 for glass.
Rare spelling using both kanji 硝子 and 球.
Compound of ガラス (glass, from Dutch 'glas') and 玉 (ball, bead).