Hello Don. If this is in an area of subduction rocks...the Franciscan Complex, then that may be a "knocker". It could be made of seafloor volcanic rock. In the back-country north of Santa Barbara, these are overlain by deep-sea cherts...banded in fine layers and folded...kind of a pretty rock. You get chert deposited in very deep water because the Calcium Carbonate that limestone is made up of dissolves so only the silica is left. A decade and a half ago we were on a field trip along the San Andreas fault and we saw knockers. A female geologists was offended by the name. She assumed that they were named for a resemblance to female breasts. When she said that, it has not occurred to me...I had seen them knocking around in a big jumble of rocks during subduction and then when the earth spat the rocks back out when the subducted rocks were captured and brought closer to the surface....

Gee must be something wrong with me when I think cherts are "pretty" and fail to note the other significance of the word "knockers".
Chris