Thallium is known as the "poisoner's poison". Ingesting a very small quantity of thallium can kill a person almost undetectably. Thallium was used in rat poison, but that practice has been discontinued.
For safety reasons, the piece shown here is not solid thallium, but a thin foil of thallium bonded to a lead planchet. The struck medal is then permanently encased in acrylic. So, the obverse has the whitish appearance of thallium, while the reverse has the darker color of lead.