conversation piece

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An unusual object that arouses comment or interest.
• noun A genre painting, popular especially in the 18th century, depicting a group of fashionable people.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An unusual item, such as a painting, or a tabletop knick-knack which encourages comment from visitors, and so helps to break the ice.
• noun A painting of a group of people engaged in conversation or some other activity, popular in 18th century.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something interesting that stimulates conversation