Rubidium "whiskers" in a vapor cell
  Crystals of metallic rubidium are observed ``growing'' from paraffin coating of buffer-gas-free glass vapor cells. The crystals have uniform square cross-section, $\approx 30 \mu$m on the side, and reach several mm in length. 
M. V. Balabas, A. O. Sushkov, and D. Budker