A frequently-famous but officially unrecognized group of stars - the well-known Big Dipper is not a constellation but (as an element of Ursa Major) an asterism.

When impurities (inclusions) in a gemstone line up with the crystal structure of the stone in such a way as to make an asterisk shape show up in a different color against the main color of the stone. "Star" gems (such as star sapphires) are usually cut as cabochons rather than faceted. Related to chatoyancy and aventurescence.

As`ter*ism (#), n. [Gr. , fr. star; cf. F. ast'erisme.]

1. Astron. (a)

A constellation.

A small cluster of stars.

2. Printing (a)

An asterisk, or mark of reference.

Three asterisks placed in this manner, &asterism;, to direct attention to a particular passage.

3. Crystallog.

An optical property of some crystals which exhibit a star-shaped by reflected light, as star sapphire, or by transmitted light, as some mica.

© Webster 1913.