A diamond is the hardest natural mineral. It is made entirely of Carbon in a tetrahedral struvture. Diamonds are usual colorless, but some contain colors due to impurities in the stone.
Topaz is the common name for Aluminum silicate fluoride hydroxide. It isusually golden brown to yellow but may be red or green depending on the impurities. Blue topaz is usually artificially produced.
Citrine, a form of quartz looks similar to topaz and is sometimes sold under the name 'topaz'. Citrine however is much less valuable.