Q.
What is the reason for placing small stones on the matzeiva after
the unveiling ceremony? If you can't find any stones, is it permitted
to take some from another kever or is that stealing?
A.
The S'hloh, (Rosh Hashana) Eliahu Rabba (224: 7). Be'er Heitev (O.H.
ibid. 8) mention the tradition and explain that the reason is
honouring the niftar by leaving a mark that shows one visited the
kever. They also mention that even leaving some grass leaves is
acceptable.
Horav
Shlomo Miller's Shlit'a opinion is that even if one may not
transgress the prohibition of stealing by removing a small stone left
on another matzeiva, as the one who placed it there may not have had
the intention of acquiring it when he pick it up and he is also
leaving it in a place that is open and hefker, it is still improper
to do so.
Rabbi
A. Bartfeld as revised by Horav Shlomo Miller Shlit”a