Sea urchin spines are porous, single crystal Mg-rich calcite [(Mg, Ca)(CO3)] with a three dimensional meshwork architecture. The crystallographic orientation of large (similar to 10 cm in length) spines from Heterocentrotus trigonarius was determined using the X-ray back-reflection Laue technique; the long axis of all spines is parallel to the [00.1] direction of calcite. The internal structure was studied using SEM and TEM. The macropore structure dominated the SEM images, whereas many small (similar to 80 nm) protein occlusions can be observed within the single crystal spines using TEM. The spines appeared crystallographically perfect in the TEM. The bend strengths of a group of spines were also determined and varied between similar to 13 and 41 MPa.