get related products by taxonomy

functions.php

function related_product_id_by_tax( $id, $tax ) {
        $transient_name = 'related_' . $id . '_' . $tax;
        $transient_time = 60 * 60 * 12; // 12 hours
        if ( false === ( $transient_results = get_transient( $transient_name ) ) ) {
                $tags_array = array();
                $tags       = wp_get_post_terms( $id, $tax );
                foreach ( $tags as $tag ) {
                        $tags_array[] .= $tag->term_id;
                }
                $args  = array(
                        'post_type'      => 'product',
                        'post__not_in'   => array( $id ),
                        'posts_per_page' => 3,
                        'fields'         => 'ids',
                        'orderby'        => 'rand',
                        'tax_query'      => array(
                                array(
                                        'taxonomy' => $tax,
                                        'field'    => 'id',
                                        'terms'    => $tags_array,
                                        'operator' => 'IN',
                                ),
                        ),
                );
                $query = new WP_Query( $args );
                set_transient( $transient_name, $query->posts, $transient_time );
                return $query->posts;
        } else {
                return $transient_results;
        }
}

related.php

<?php
/**** override: this file goes in theme/woocommerce/single-product ****/
/**
 * Related Products
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product/related.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see         https://docs.woocommerce.com/document/template-structure/
 * @author      WooThemes
 * @package     WooCommerce/Templates
 * @version     3.0.0
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

if ( $related_products ) : ?>

	<section class="related products">

		<h2><?php esc_html_e( 'Related products', 'woocommerce' ); ?></h2>
		<?php
		$ids = related_product_id_by_tax( get_the_ID(), 'stones' );
		?>
		<?php woocommerce_product_loop_start(); ?>

			<?php foreach ( $ids as $an_id ) : ?>

				<?php
				 	$post_object = get_post( $an_id );

					setup_postdata( $GLOBALS['post'] =& $post_object );

					wc_get_template_part( 'content', 'product' );
					?>

			<?php endforeach; ?>

		<?php woocommerce_product_loop_end(); ?>

	</section>

<?php
endif;

wp_reset_postdata();