The term 'Tough Gemstone' can be summarized by the crossword solution of JADE, which is a 4 letter word. JADE is a mineral composed of two different silicate materials, jadeite and nephrite, that are found in many parts of the world. It is known for its toughness and durability, making it a popular choice for jewelry and other decorative items. JADE has been used for centuries in various cultures, and is still highly valued today.