Online jewelry and watch sales are down 17 percent in a year-over-year comparison for the first 19 days of December, according to digital tracking firm comScore.

Jewelry and watches is one of the poorest performing categories this holiday season, according to the Reston, Va.-based company. Office supplies (down 19%) and music, movies, and videos (down 24%) are the categories that have performed worse during December, comScore reports.

The fastest growing product categories during the period (from Dec. 1 through Dec. 19) were Sport & Fitness (up 31%), Books & Magazines (up 18%), and Video Games, Consoles & Accessories (up 17%). Apparel & Accessories, the second largest online retail category by dollar sales, has seen sales grow strongly (up 15%) as a result of retailers’ aggressive discounting and unseasonably cold weather across many parts of the country, comScore reports.

Following a strong week of Cyber Monday, that was driven by large discounts on products such as flat screen TVs, sales of consumer electronics have slowed significantly in recent weeks, according to comScore.

So far, $24.03 billion has been spent online for the first 49 days of the November – December 2008 holiday season, (through Dec. 19), down 1 percent versus the corresponding days last year. The most recent week of the season has seen several strong online spending days, including Dec. 15 with $859 million in spending to rank as the second heaviest day of the season, behind Dec. 9 when $887 million was spent.

“With five fewer days of holiday shopping between Thanksgiving and Christmas this year, there is increasing pressure on consumers to make their holiday purchases in time for Christmas,” said comScore chairman Gian Fulgoni. “As a result, we’ve seen online shoppers continue to spend heavily even later into the season, with the most recent week including four of the top ten spending days of the season. Despite this positive development, the combination of the compressed holiday schedule and the challenging economic situation faced by many consumers means that retailers have their work cut out for them this season.”

This year’s holiday season from Thanksgiving through Dec. 19 has seen average online spending per day of $643 million, 5 percent higher than last year. However, the core holiday shopping season between Thanksgiving and Christmas this year is actually 16 percent shorter than last year, which underscores one of the key challenges facing retailers this season: an inability to increase daily sales at a rate sufficient to compensate for fewer shopping days.

JCK