Which amidst the following is not a gem stone ?

A gemstone or gem (also called a precious or semiprecious stone, a fine gem, or jewel) is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments. However certain rocks (such as lapis lazuli) and organic materials (such as amber or jet) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Topaz, Opal and Pearl are gemstones but Cat’s-eye is not a gem stone.