I did not find the particulair Midrash in Bereishis Rabbah. But, the Midrash in Koheles Rabbah (the Midrash on Ecclessiastes) explains that the luz bone is the nut-sized bone at the top of the spinal column.
“The almond tree will blossom” – Rabbi Levi said: This is the
nut-sized bone at the top of the spinal column. Hadrian, may his bones
be crushed and his name expunged, asked Rabbi Yehoshua ben Ḥananya; he
said to him: ‘From where does a person blossom in the future?’ He said
to him: ‘From the nut-sized bone at the top of the spinal column.’
[Hadrian] said to him: ‘Show me.’ What did [Rabbi Yehoshua] do? He
brought him the nut-sized bone at the top of the spinal column. He
placed it in water, but it did not dissolve; in fire, but it did not
burn; in a mill, but it was not ground. He placed in on an anvil and
struck it with a hammer. The anvil split, the hammer split, but it was
to no avail.