Amethyst cluster, February birthstone

A cluster of naturally coloured purple amethyst crystals from Brazil. This is actually the end of a stalactite or stalagmite.  Do you know how to tell which is which in situ? A stalactite has to hold on tight, a stalagmite might reach the roof.  Amethyst is a February birthstone.

1 inch (2.5 cm) wide, 15/16 inch (2.4 cm) deep and 1/2 inch (1.3 cm) tall.