You can do gem work with acrylic, glass or crystal rhinestones. 

Purchase acrylic, glass, or crystal flat-backed rhinestones. These are often available at craft supply stores.  They're colorful, and make simple patterns look special.  Gems, some body giltter, and gilding make a henna pattern dazzle.

Apply gems with Liquid New Skin, spirit gum, or eyelash glue.

Dab the adhesive on the skin, then dab it on the back of the gem, then stick the gem on the skin.
Acrylic rhinestones are very inexpensive and easily available at craft stores.

Most adhesives disolve the silver back of the acrylic gem, and make it look cloudy instead of sparkly.

Swarovski crystals look like real gemstones. 

They cost more than other rhinestones, but are are dazzling, and keep their sparkle. They don't go cloudy from the adhesive.  Use these for your very finest work!

You can get acrylic and glass flat-backed rhinestones at most craft and hobby stores.