Question

Topaz, opal, agate, and onyx are precious and beautiful stones found in nature. All these are different forms of:

• A. silica
• B. soda
• C. carbon
• D. lime

Answer 1

The correct option is A silica

• Topaz is a rare silicate mineral with a chemical composition of $A{l}_{2}Si{O}_4(FOH{)}_2$. It usually forms in fractures and cavities of igneous rocks such as pegmatite and rhyolite, late in their cooling history.
  
• Opal is a non-crystalline form of the mineral silica and is formed from amorphous "balls" or "lumps" of silica. So the opal is a very unique gem compared with others, like diamonds or a ruby which is formed crystalline. This is the reason why variation of colours are in opals.
  
• Agate is a banded microcrystalline form of the mineral Quartz, and does not occur in visible crystals. It occurs in nodules, in massive form, as botryoidal, mammilary, and stalactitic formations, as smooth rounded pebbles, as amygdules, and as the linings of geodes.
  
• Onyx is formed of bands of chalcedony in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.
  
• Hence option A is correct answer.