Question

Topaz, opal, agate and onyx are precious and beautiful stones found in nature. All of these are different forms of:
A.Silica
B.Soda
C.Carbon
D.Lime

Answer 1

Hint: To answer this question, recall the concept of precious stones. Topaz, opal, agate and onyx are all crystals formed in caves. They all compose the same element found in quartz glass.

Complete step by step answer:
Topaz is a rare silicate mineral with a chemical composition of \[A{l_{2}}Si{O_4}{\left( {FOH} \right)_2}\] ​. 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 the amorphous form of silica. So the opal is a very unique gem compared with others, like diamonds or a ruby which is formed by crystalline. This is the reason why the variation of colours is 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 in different formations, as smooth rounded pebbles and as the linings of geodes.
Onyx is formed of bands of chalcedony in alternating colours. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and morganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.

Hence, the correct answer to this question is option A.

Note:
Some other important types of silicates found in nature include:
Orthosilicates (or Nesosilicates): The orthosilicate ion is the strong conjugate base of weak orthosilicate acid as well as it will not persist in aqueous solutions. In nature, orthosilicates are rare and only found with cations which form highly insoluble salts.
Pyro silicate (or Sorosilicates)
Cyclic silicates (or Ring silicates)
Chain silicates (or pyroxenes)
Double chain silicate (or amphiboles)
Sheet or phyllosilicates
Three dimensional (or tecto) silicates