Amethyst and citrine are both members of the quartz family along with smoky and rose quartz, agate, bloodstone and more. A diamond is by far the hardest of any gemstone. Platinum is the hardest of the pure precious metals. Much harder than PURE gold and silver. Although platinum is the hardest of the pure precious metals, certain gold alloys, especially 14 karat white gold, are much harder.
Tourmaline occurs in pink, red, blue, green, brown, and many colors in-between.
A cabochon is a smooth rounded stone with no facets. Almost any gemstone can be cut as a cabochon. Almost all opaque gems are cabochon cut. Platinum is non-reactive with virtually all chemicals and materials. It is completely non-reactive to an individual's body chemistry.