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Tenacity describes how a mineral behaves when scratched and bent. Similar to a hardness test, the mineral is scratched with a steel needle. If the scratch dust flies away, the mineral is brittle. This is the case for 99% of minerals. Mild tenacity is present when the scratch dust does not fly away but remains next to the scratch mark. Examples of this are galena and stibnite. With cutable tenacity, the needle penetrates but leaves no dust. Examples are acanthite, native gold, and bismuth. Acanthite and native gold can also be hammered into flakes; they are malleable. Other minerals are elastically flexible; they return to their original shape after being bent. Still others are inelastically flexible and remain in their new shape after being bent, such as gypsum or stibnite (see image).
Figure 5: The mineral stibnite is one of the special cases that exhibit inelastic-flexible tenacity.