black spots on travertine shower

Q. My master bath was remodeled approx. 8 months ago and we used honed and tumbled travertine. It was professionally installed and moisture barrier was used in shower. Approx a month ago, I noticed a few black "dots" on the floor of the shower (they are 2x2 tumbled durango), just below the shower head. I cleaned the travertine with Tile Guard, which appears to be just an everyday cleaner for stone. It will not remove these spots and I've noticed a few more have appeared. I have scrubbed with a toothbrush and still no change. Do you have any idea what this is or what I can use on it? I'm concerned it's mold and if that's the case, have no idea what to do. I could send pics if that would be helpful. I'm so glad to have found your site and hope you will be able to resolve my problem. I look forward to your response. Thanks!

A. It does sound like the black spots could be mold or mildew. To test this take a drop of bleach and place it on one of the black spots and let it sit for a minute or two. If the black spot disappears then it is mold. Clean the remaining spots with bleach and rinse really well. Let the stone dry and seal it with a good impregnating stone sealer.

Fred Hueston
Chief Technical Director, SurpHaces
President, Stone Forensics