What cable is needed to connect a Crucial X8 500 GB SSD to a ThunderBolt port on my iMac?

According to the Crucial x8 product page you should have received a USB-C to USB-A Adapter with your SSD to connect it to an older computer such as your iMac which only has a USB-A port. The speed of the device will depend on the USB3 specifications of your iMac's USB port.

https://www.crucial.com/products/ssd/x8-portable-ssd

Here is a screenshot of the information and a picture of the SSD connected to an older laptop with just a standard USB-A port (notice the USB-C Female to USB-A Male Adapter on the laptop end of the cable):

You cannot connect a USB device to a Thunderbolt 1 or Thunderbolt 2 port. You can connect a USB-C device to a Thunderbolt 3 port as long as it supports the USB protocol. Apple's USB-C Macs support both Thunderbolt 3 and USB3 (some support USB4) on the Mac's USB-C ports. It can be difficult trying to make sense of it all.

USB-C can be physically plugged into a Thunderbolt-3 or a dedicated USB-C port. These are not on your 2013 iMac, and no cabling will change that.

You can use the alternate USB-A cable supplied with the X8 to plug directly into one of your existing USB-3 ports on the iMac. These ports offer 5Gbps/500MB/s (theoretical) and the Crucial X8 is rated at 1050 MB/s, so it won't run flat out until you upgrade your Mac hardware to something that offers a ThunderBolt-3/USB 3.1 Gen 2 port.

You must use USB. This will give you ~420 MB/s.

For SSD recommendations and setup instructions, please read: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack