Like most other objects, minerals differ in their abilities to absorb, reflect and transmit light at various wavelengths, and therefore will produce different colors. Color is also an important property in determining the value of gemstones. For example, the gemstones ruby and sapphire are both crystals of the mineral corundum, and are made composed of aluminum oxide. Usually a colorless crystal, corundum will produce different colors by having traces of iron, titanium and chromium ions; traces of chromium (cr3+) ions that replaced al3+ ions will produce red corundum (ruby); traces of iron and titanium ions result in blue color, producing blue sapphire. These ions that give jewels their colors are called “color or pigment ions”. Variations of these trace ions in the same crystal will produce brilliant colors - which is one of the reason why gemstones are so fascinating.