Allen and Greenough's New Latin Grammar, section 32

Heavy Construction

The Allen and Greenough is still under construction; so some links may not work quite the way you would expect.

32.

Names of Female beings, of Cities, Countries, Plants, Trees, and Gems, of many Animals (especially Birds), and of most abstract Qualities, are feminine: -

máter, mother; Iúlia, Julia; Róma, Rome; Ítalia, Italy; rosa, rose; pínus, pine; sapphírus, sapphire; anas, duck; véritás, truth.

a. Some names of Towns and Countries are masculine: as, Sulmó, Gabií (plur.); or neuter, as, Tarentum, Illyricum.

b. A few names of Plants and Gems follow the gender of their termination: as, centauréum (N.), centaury; acanthus (M.), bearsfoot; opalus (M.) opal.

NOTE: The gender of most of the above may also be recognized by the terminations, according to the rules given under the several declensions. The names of Roman women were usually feminine adjectives denoting their g=ens or house (see §108. b).