Prehnite is a silicate mineral, not familiar to most buyers, and it is named after the Dutch colonel who first described it from material collected at the Cape of Good Hope in the eighteenth century. Its colour is a soft, slightly milky green.
Most natural prehnite carries dark green epidote inclusions, and a strand with none at all is worth a second look — it may simply be a different stone sold under the name, which happens with the less common minerals. It is a moderately soft material and will scratch against metal.