Which one of the following gemstones contains beryllium metal?

The correct answer is option 2 i.e Emerald 

Explanation of some of the Gemstones are:

Topaz

• It is a rare silicate mineral with a chemical composition of Al₂SiO₄(F,OH)₂
• It has a hardness of 8 on the Mohs Hardness scale.
• It is found where rocks like Pegmatite and Rhyolite are formed. 

Emerald 

• They are gem-quality specimens of the beryl mineral family with a rich, distinctly green colour.
• It has a hardness of 7.5 to 8 on the Mohs Hardness scale.
• Chemical composition of Be₃Al₂(SiO₃)₆. When pure, beryl is colourless and known as "Goshenite''.
• Trace amounts of Chromium or Vanadium in the mineral cause it to develop a green colour.
• They are found in igneous, metamorphic and sedimentary rocks.

Ruby

• They are gem corundum with a dominant Red colour (due to the presence of Chromium).
• It has a hardness of 9 on the Mohs Hardness scale.

Blue sapphire

• Trace amounts of Iron and Titanium can produce a blue colour in corundum.
• Blue corundums are known as Sapphires.