I scraped off a bit of my tripoli bar, got rid of the matrix with hot water, dried the powdery stuff and peered at it with a 40x and a 100x lens in my microscope. There were irregularly shaped particles, but no diatoms I'm afraid.
There are several kinds of tripoli, one being in fact
diatomaceous earth or terra silicea. In 1 cubic centimeter,
weighing .25 grams, there are about 1 thousand millions fossil
skeletons. I’ve got a photograph with a 1:1800 enlargement, and
the largest single diatomee is about 1 inch across.
American tripoli from IL and TN doesn’t contain diatomees, but
consists of 92% amorphous quartz, 1.3% iron (III) oxide, 3.6%
aluminium oxide and some other additions, colour off white to
yellow, mixed with tallow, other fats and C17H35COOH (stearine
acid? Don’t know the English name) for buffing.
German tripoli contains 78%-89% diatomaceous earth, 0.35%-1.8%
iron (III) oxide, 1.82%-5.28% aluminium oxide, is softer than
Rottenstone is a substiute for tripoli, 80%-85% silicon dioxide,
10% aluminium oxide, yellow-red to yellow-grey colour.
Artificial tripoli made of pure silicon dioxide, cristalline
quartz and sand.