Taken from the Wikipedia entry on ‘brain sand':
Corpora arenacea (or brain sand) are calcified structures in the pineal gland and other areas of the brain such as the choroid plexus. Older organisms have numerous corpora arenacea, whose function, if any, is unknown. Concentrations of “brain sand” increase with age, so the pineal gland becomes increasingly visible on X-rays over time, usually by the third or fourth decade. They are sometimes used as anatomical landmarks in radiological examinations.
Chemical analysis shows that they are composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate. Recently, calcite deposits have been described as well.
French philosopher Ren√© Descartes famously concluded that the mind and the brain existed as entirely separate entities (a position now known as Cartesian dualism) and believed that pineal gland was the point at which the two interacted.
This was due to the fact that that, unlike most other structures in the brain, there is only one pineal gland and it is located exactly along the midline.
As Descartes largely thought of the mind and soul as the same thing, I’d like to think he would have called these calcified particles ‘soul sand’ had he known about them.
If you want some more details on ‘brain sand’, of which we know very little, this large abstract of a scientific study has a wealth of information.